Enhance TLS Emulator with ALPN Support and Add Adversarial Tests

- Modified `build_emulated_server_hello` to accept ALPN (Application-Layer Protocol Negotiation) as an optional parameter, allowing for the embedding of ALPN markers in the application data payload.
- Implemented logic to handle oversized ALPN values and ensure they do not interfere with the application data payload.
- Added new security tests in `emulator_security_tests.rs` to validate the behavior of the ALPN embedding, including scenarios for oversized ALPN and preference for certificate payloads over ALPN markers.
- Introduced `send_adversarial_tests.rs` to cover edge cases and potential issues in the middle proxy's send functionality, ensuring robustness against various failure modes.
- Updated `middle_proxy` module to include new test modules and ensure proper handling of writer commands during data transmission.

src/ip_tracker.rs

@@ -7,8 +7,9 @@ use std::net::IpAddr;
 use std::sync::Arc;
 use std::sync::atomic::{AtomicU64, Ordering};
 use std::time::{Duration, Instant};
+use std::sync::Mutex;
 
-use tokio::sync::RwLock;
+use tokio::sync::{Mutex as AsyncMutex, RwLock};
 
 use crate::config::UserMaxUniqueIpsMode;
 
@@ -21,6 +22,8 @@ pub struct UserIpTracker {
     limit_mode: Arc<RwLock<UserMaxUniqueIpsMode>>,
     limit_window: Arc<RwLock<Duration>>,
     last_compact_epoch_secs: Arc<AtomicU64>,
+    pub(crate) cleanup_queue: Arc<Mutex<Vec<(String, IpAddr)>>>,
+    cleanup_drain_lock: Arc<AsyncMutex<()>>,
 }
 
 impl UserIpTracker {
@@ -33,6 +36,67 @@ impl UserIpTracker {
             limit_mode: Arc::new(RwLock::new(UserMaxUniqueIpsMode::ActiveWindow)),
             limit_window: Arc::new(RwLock::new(Duration::from_secs(30))),
             last_compact_epoch_secs: Arc::new(AtomicU64::new(0)),
+            cleanup_queue: Arc::new(Mutex::new(Vec::new())),
+            cleanup_drain_lock: Arc::new(AsyncMutex::new(())),
+        }
+    }
+
+
+    pub fn enqueue_cleanup(&self, user: String, ip: IpAddr) {
+        match self.cleanup_queue.lock() {
+            Ok(mut queue) => queue.push((user, ip)),
+            Err(poisoned) => {
+                let mut queue = poisoned.into_inner();
+                queue.push((user.clone(), ip));
+                self.cleanup_queue.clear_poison();
+                tracing::warn!(
+                    "UserIpTracker cleanup_queue lock poisoned; recovered and enqueued IP cleanup for {} ({})",
+                    user,
+                    ip
+                );
+            }
+        }
+    }
+
+    pub(crate) async fn drain_cleanup_queue(&self) {
+        // Serialize queue draining and active-IP mutation so check-and-add cannot
+        // observe stale active entries that are already queued for removal.
+        let _drain_guard = self.cleanup_drain_lock.lock().await;
+        let to_remove = {
+            match self.cleanup_queue.lock() {
+                Ok(mut queue) => {
+                    if queue.is_empty() {
+                        return;
+                    }
+                    std::mem::take(&mut *queue)
+                }
+                Err(poisoned) => {
+                    let mut queue = poisoned.into_inner();
+                    if queue.is_empty() {
+                        self.cleanup_queue.clear_poison();
+                        return;
+                    }
+                    let drained = std::mem::take(&mut *queue);
+                    self.cleanup_queue.clear_poison();
+                    drained
+                }
+            }
+        };
+
+        let mut active_ips = self.active_ips.write().await;
+        for (user, ip) in to_remove {
+            if let Some(user_ips) = active_ips.get_mut(&user) {
+                if let Some(count) = user_ips.get_mut(&ip) {
+                    if *count > 1 {
+                        *count -= 1;
+                    } else {
+                        user_ips.remove(&ip);
+                    }
+                }
+                if user_ips.is_empty() {
+                    active_ips.remove(&user);
+                }
+            }
         }
     }
 
@@ -118,6 +182,7 @@ impl UserIpTracker {
     }
 
     pub async fn check_and_add(&self, username: &str, ip: IpAddr) -> Result<(), String> {
+        self.drain_cleanup_queue().await;
         self.maybe_compact_empty_users().await;
         let default_max_ips = *self.default_max_ips.read().await;
         let limit = {
@@ -194,6 +259,7 @@ impl UserIpTracker {
     }
 
     pub async fn get_recent_counts_for_users(&self, users: &[String]) -> HashMap<String, usize> {
+        self.drain_cleanup_queue().await;
         let window = *self.limit_window.read().await;
         let now = Instant::now();
         let recent_ips = self.recent_ips.read().await;
@@ -214,6 +280,7 @@ impl UserIpTracker {
     }
 
     pub async fn get_active_ips_for_users(&self, users: &[String]) -> HashMap<String, Vec<IpAddr>> {
+        self.drain_cleanup_queue().await;
         let active_ips = self.active_ips.read().await;
         let mut out = HashMap::with_capacity(users.len());
         for user in users {
@@ -228,6 +295,7 @@ impl UserIpTracker {
     }
 
     pub async fn get_recent_ips_for_users(&self, users: &[String]) -> HashMap<String, Vec<IpAddr>> {
+        self.drain_cleanup_queue().await;
         let window = *self.limit_window.read().await;
         let now = Instant::now();
         let recent_ips = self.recent_ips.read().await;
@@ -250,11 +318,13 @@ impl UserIpTracker {
     }
 
     pub async fn get_active_ip_count(&self, username: &str) -> usize {
+        self.drain_cleanup_queue().await;
         let active_ips = self.active_ips.read().await;
         active_ips.get(username).map(|ips| ips.len()).unwrap_or(0)
     }
 
     pub async fn get_active_ips(&self, username: &str) -> Vec<IpAddr> {
+        self.drain_cleanup_queue().await;
         let active_ips = self.active_ips.read().await;
         active_ips
             .get(username)
@@ -263,6 +333,7 @@ impl UserIpTracker {
     }
 
     pub async fn get_stats(&self) -> Vec<(String, usize, usize)> {
+        self.drain_cleanup_queue().await;
         let active_ips = self.active_ips.read().await;
         let max_ips = self.max_ips.read().await;
         let default_max_ips = *self.default_max_ips.read().await;
@@ -301,6 +372,7 @@ impl UserIpTracker {
     }
 
     pub async fn is_ip_active(&self, username: &str, ip: IpAddr) -> bool {
+        self.drain_cleanup_queue().await;
         let active_ips = self.active_ips.read().await;
         active_ips
             .get(username)

src/ip_tracker_regression_tests.rs

@@ -448,3 +448,172 @@ async fn concurrent_reconnect_and_disconnect_preserves_non_negative_counts() {
 
     assert!(tracker.get_active_ip_count("cc").await <= 8);
 }
+
+#[tokio::test]
+async fn enqueue_cleanup_recovers_from_poisoned_mutex() {
+    let tracker = UserIpTracker::new();
+    let ip = ip_from_idx(99);
+    
+    // Poison the lock by panicking while holding it
+    let result = std::panic::catch_unwind(|| {
+        let _guard = tracker.cleanup_queue.lock().unwrap();
+        panic!("Intentional poison panic");
+    });
+    assert!(result.is_err(), "Expected panic to poison mutex");
+    
+    // Attempt to enqueue anyway; should hit the poison catch arm and still insert
+    tracker.enqueue_cleanup("poison-user".to_string(), ip);
+    
+    tracker.drain_cleanup_queue().await;
+    
+    assert_eq!(tracker.get_active_ip_count("poison-user").await, 0);
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 4)]
+async fn mass_reconnect_sync_cleanup_prevents_temporary_reservation_bloat() {
+    // Tests that synchronous M-01 drop mechanism protects against starvation
+    let tracker = Arc::new(UserIpTracker::new());
+    tracker.set_user_limit("mass", 5).await;
+    
+    let ip = ip_from_idx(42);
+    let mut join_handles = Vec::new();
+
+    // 10,000 rapid concurrent requests hitting the same IP limit
+    for _ in 0..10_000 {
+        let tracker_clone = tracker.clone();
+        join_handles.push(tokio::spawn(async move {
+            if tracker_clone.check_and_add("mass", ip).await.is_ok() {
+                // Instantly enqueue cleanup, simulating synchronous reservation drop
+                tracker_clone.enqueue_cleanup("mass".to_string(), ip);
+                // The next caller will drain it before acquiring again
+            }
+        }));
+    }
+
+    for handle in join_handles {
+        let _ = handle.await;
+    }
+
+    // Force flush
+    tracker.drain_cleanup_queue().await;
+    assert_eq!(tracker.get_active_ip_count("mass").await, 0, "No leaked footprints");
+}
+
+#[tokio::test]
+async fn adversarial_drain_cleanup_queue_race_does_not_cause_false_rejections() {
+    // Regression guard: concurrent cleanup draining must not produce false
+    // limit denials for a new IP when the previous IP is already queued.
+    let tracker = Arc::new(UserIpTracker::new());
+    tracker.set_user_limit("racer", 1).await;
+    let ip1 = ip_from_idx(1);
+    let ip2 = ip_from_idx(2);
+
+    // Initial state: add ip1
+    tracker.check_and_add("racer", ip1).await.unwrap();
+
+    // User disconnects from ip1, queuing it
+    tracker.enqueue_cleanup("racer".to_string(), ip1);
+
+    let mut saw_false_rejection = false;
+    for _ in 0..100 {
+        // Queue cleanup then race explicit drain and check-and-add on the alternative IP.
+        tracker.enqueue_cleanup("racer".to_string(), ip1);
+        let tracker_a = tracker.clone();
+        let tracker_b = tracker.clone();
+
+        let drain_handle = tokio::spawn(async move {
+            tracker_a.drain_cleanup_queue().await;
+        });
+        let handle = tokio::spawn(async move {
+            tracker_b.check_and_add("racer", ip2).await
+        });
+
+        drain_handle.await.unwrap();
+        let res = handle.await.unwrap();
+        if res.is_err() {
+            saw_false_rejection = true;
+            break;
+        }
+
+        // Restore baseline for next iteration.
+        tracker.remove_ip("racer", ip2).await;
+        tracker.check_and_add("racer", ip1).await.unwrap();
+    }
+
+    assert!(
+        !saw_false_rejection,
+        "Concurrent cleanup draining must not cause false-positive IP denials"
+    );
+}
+
+#[tokio::test]
+async fn poisoned_cleanup_queue_still_releases_slot_for_next_ip() {
+    let tracker = UserIpTracker::new();
+    tracker.set_user_limit("poison-slot", 1).await;
+    let ip1 = ip_from_idx(7001);
+    let ip2 = ip_from_idx(7002);
+
+    tracker.check_and_add("poison-slot", ip1).await.unwrap();
+
+    // Poison the queue lock as an adversarial condition.
+    let _ = std::panic::catch_unwind(|| {
+        let _guard = tracker.cleanup_queue.lock().unwrap();
+        panic!("intentional queue poison");
+    });
+
+    // Disconnect path must still queue cleanup so the next IP can be admitted.
+    tracker.enqueue_cleanup("poison-slot".to_string(), ip1);
+    let admitted = tracker.check_and_add("poison-slot", ip2).await;
+    assert!(
+        admitted.is_ok(),
+        "cleanup queue poison must not permanently block slot release for the next IP"
+    );
+}
+
+#[tokio::test]
+async fn duplicate_cleanup_entries_do_not_break_future_admission() {
+    let tracker = UserIpTracker::new();
+    tracker.set_user_limit("dup-cleanup", 1).await;
+    let ip1 = ip_from_idx(7101);
+    let ip2 = ip_from_idx(7102);
+
+    tracker.check_and_add("dup-cleanup", ip1).await.unwrap();
+    tracker.enqueue_cleanup("dup-cleanup".to_string(), ip1);
+    tracker.enqueue_cleanup("dup-cleanup".to_string(), ip1);
+    tracker.enqueue_cleanup("dup-cleanup".to_string(), ip1);
+
+    tracker.drain_cleanup_queue().await;
+
+    assert_eq!(tracker.get_active_ip_count("dup-cleanup").await, 0);
+    assert!(
+        tracker.check_and_add("dup-cleanup", ip2).await.is_ok(),
+        "extra queued cleanup entries must not leave user stuck in denied state"
+    );
+}
+
+#[tokio::test]
+async fn stress_repeated_queue_poison_recovery_preserves_admission_progress() {
+    let tracker = UserIpTracker::new();
+    tracker.set_user_limit("poison-stress", 1).await;
+    let ip_primary = ip_from_idx(7201);
+    let ip_alt = ip_from_idx(7202);
+
+    tracker.check_and_add("poison-stress", ip_primary).await.unwrap();
+
+    for _ in 0..64 {
+        let _ = std::panic::catch_unwind(|| {
+            let _guard = tracker.cleanup_queue.lock().unwrap();
+            panic!("intentional queue poison in stress loop");
+        });
+
+        tracker.enqueue_cleanup("poison-stress".to_string(), ip_primary);
+
+        assert!(
+            tracker.check_and_add("poison-stress", ip_alt).await.is_ok(),
+            "poison recovery must preserve admission progress under repeated queue poisoning"
+        );
+
+        tracker.remove_ip("poison-stress", ip_alt).await;
+        tracker.check_and_add("poison-stress", ip_primary).await.unwrap();
+    }
+}

src/protocol/tls.rs

@@ -34,6 +34,9 @@ pub const TIME_SKEW_MIN: i64 = -2 * 60; // 2 minutes before
 pub const TIME_SKEW_MAX: i64 = 2 * 60;  // 2 minutes after
 /// Maximum accepted boot-time timestamp (seconds) before skew checks are enforced.
 pub const BOOT_TIME_MAX_SECS: u32 = 7 * 24 * 60 * 60;
+/// Hard cap for boot-time compatibility bypass to avoid oversized acceptance
+/// windows when replay TTL is configured very large.
+pub const BOOT_TIME_COMPAT_MAX_SECS: u32 = 2 * 60;
 
 // ============= Private Constants =============
 
@@ -66,6 +69,7 @@ pub struct TlsValidation {
     /// Client digest for response generation
     pub digest: [u8; TLS_DIGEST_LEN],
     /// Timestamp extracted from digest
+    
     pub timestamp: u32,
 }
 
@@ -121,6 +125,7 @@ impl TlsExtensionBuilder {
     }
 
     /// Build final extensions with length prefix
+    
     fn build(self) -> Vec<u8> {
         let mut result = Vec::with_capacity(2 + self.extensions.len());
         
@@ -135,7 +140,7 @@ impl TlsExtensionBuilder {
     }
     
     /// Get current extensions without length prefix (for calculation)
-    #[allow(dead_code)]
+    
     fn as_bytes(&self) -> &[u8] {
         &self.extensions
     }
@@ -251,6 +256,7 @@ impl ServerHelloBuilder {
 /// Returns validation result if a matching user is found.
 /// The result **must** be used — ignoring it silently bypasses authentication.
 #[must_use]
+
 pub fn validate_tls_handshake(
     handshake: &[u8],
     secrets: &[(String, Vec<u8>)],
@@ -266,9 +272,9 @@ pub fn validate_tls_handshake(
 
 /// Validate TLS ClientHello and cap the boot-time bypass by replay-cache TTL.
 ///
-/// A boot-time timestamp is only accepted when it falls below both
+/// A boot-time timestamp is only accepted when it falls below all three
-/// `BOOT_TIME_MAX_SECS` and the configured replay window, preventing timestamp
+/// bounds: `BOOT_TIME_MAX_SECS`, configured replay window, and
-/// reuse outside replay cache coverage.
+/// `BOOT_TIME_COMPAT_MAX_SECS`, preventing oversized compatibility windows.
 #[must_use]
 pub fn validate_tls_handshake_with_replay_window(
     handshake: &[u8],
@@ -292,7 +298,9 @@ pub fn validate_tls_handshake_with_replay_window(
     let boot_time_cap_secs = if ignore_time_skew {
         0
     } else {
-        BOOT_TIME_MAX_SECS.min(replay_window_u32)
+        BOOT_TIME_MAX_SECS
+            .min(replay_window_u32)
+            .min(BOOT_TIME_COMPAT_MAX_SECS)
     };
 
     validate_tls_handshake_at_time_with_boot_cap(
@@ -312,6 +320,7 @@ fn system_time_to_unix_secs(now: SystemTime) -> Option<i64> {
     i64::try_from(d.as_secs()).ok()
 }
 
+
 fn validate_tls_handshake_at_time(
     handshake: &[u8],
     secrets: &[(String, Vec<u8>)],
@@ -437,7 +446,7 @@ pub fn build_server_hello(
     session_id: &[u8],
     fake_cert_len: usize,
     rng: &SecureRandom,
-    _alpn: Option<Vec<u8>>,
+    alpn: Option<Vec<u8>>,
     new_session_tickets: u8,
 ) -> Vec<u8> {
     const MIN_APP_DATA: usize = 64;
@@ -459,8 +468,27 @@ pub fn build_server_hello(
         0x01,       // CCS byte
     ];
     
-    // Build fake certificate (Application Data record)
+    // Build first encrypted flight mimic as opaque ApplicationData bytes.
-    let fake_cert = rng.bytes(fake_cert_len);
+    // Embed a compact EncryptedExtensions-like ALPN block when selected.
+    let mut fake_cert = Vec::with_capacity(fake_cert_len);
+    if let Some(proto) = alpn.as_ref().filter(|p| !p.is_empty() && p.len() <= u8::MAX as usize) {
+        let proto_list_len = 1usize + proto.len();
+        let ext_data_len = 2usize + proto_list_len;
+        let marker_len = 4usize + ext_data_len;
+        if marker_len <= fake_cert_len {
+            fake_cert.extend_from_slice(&0x0010u16.to_be_bytes());
+            fake_cert.extend_from_slice(&(ext_data_len as u16).to_be_bytes());
+            fake_cert.extend_from_slice(&(proto_list_len as u16).to_be_bytes());
+            fake_cert.push(proto.len() as u8);
+            fake_cert.extend_from_slice(proto);
+        }
+    }
+    if fake_cert.len() < fake_cert_len {
+        fake_cert.extend_from_slice(&rng.bytes(fake_cert_len - fake_cert.len()));
+    } else if fake_cert.len() > fake_cert_len {
+        fake_cert.truncate(fake_cert_len);
+    }
+
     let mut app_data_record = Vec::with_capacity(5 + fake_cert_len);
     app_data_record.push(TLS_RECORD_APPLICATION);
     app_data_record.extend_from_slice(&TLS_VERSION);
@@ -472,8 +500,9 @@ pub fn build_server_hello(
     // Build optional NewSessionTicket records (TLS 1.3 handshake messages are encrypted;
     // here we mimic with opaque ApplicationData records of plausible size).
     let mut tickets = Vec::new();
-    if new_session_tickets > 0 {
+    let ticket_count = new_session_tickets.min(4);
-        for _ in 0..new_session_tickets {
+    if ticket_count > 0 {
+        for _ in 0..ticket_count {
             let ticket_len: usize = rng.range(48) + 48; // 48-95 bytes
             let mut record = Vec::with_capacity(5 + ticket_len);
             record.push(TLS_RECORD_APPLICATION);
@@ -678,6 +707,7 @@ pub fn is_tls_handshake(first_bytes: &[u8]) -> bool {
 }
 
 /// Parse TLS record header, returns (record_type, length)
+
 pub fn parse_tls_record_header(header: &[u8; 5]) -> Option<(u8, u16)> {
     let record_type = header[0];
     let version = [header[1], header[2]];

src/protocol/tls_security_tests.rs

@@ -300,8 +300,8 @@ fn boot_time_timestamp_accepted_without_ignore_flag() {
     // Timestamps below the boot-time threshold are treated as client uptime,
     // not real wall-clock time.  The proxy allows them regardless of skew.
     let secret = b"boot_time_test";
-    // Keep this safely below BOOT_TIME_MAX_SECS to assert bypass behavior.
+    // Keep this safely below compatibility cap to assert bypass behavior.
-    let boot_ts: u32 = BOOT_TIME_MAX_SECS / 2;
+    let boot_ts: u32 = BOOT_TIME_COMPAT_MAX_SECS.saturating_sub(1);
     let handshake = make_valid_tls_handshake(secret, boot_ts);
     let secrets = vec![("u".to_string(), secret.to_vec())];
     assert!(
@@ -663,13 +663,14 @@ fn zero_length_session_id_accepted() {
 // Boot-time threshold — exact boundary precision
 // ------------------------------------------------------------------
 
-/// timestamp = BOOT_TIME_MAX_SECS - 1 is the last value inside the boot-time window.
+/// timestamp = BOOT_TIME_COMPAT_MAX_SECS - 1 is the last value inside
+/// the runtime boot-time compatibility window.
 /// is_boot_time = true → skew check is skipped entirely → accepted even
 /// when `now` is far from the timestamp.
 #[test]
 fn timestamp_one_below_boot_threshold_bypasses_skew_check() {
     let secret = b"boot_last_value_test";
-    let ts: u32 = BOOT_TIME_MAX_SECS - 1;
+    let ts: u32 = BOOT_TIME_COMPAT_MAX_SECS - 1;
     let h = make_valid_tls_handshake(secret, ts);
     let secrets = vec![("u".to_string(), secret.to_vec())];
 
@@ -677,32 +678,48 @@ fn timestamp_one_below_boot_threshold_bypasses_skew_check() {
     // Boot-time bypass must prevent the skew check from running.
     assert!(
         validate_tls_handshake_at_time(&h, &secrets, false, 0).is_some(),
-        "ts=BOOT_TIME_MAX_SECS-1 must bypass skew check regardless of now"
+        "ts=BOOT_TIME_COMPAT_MAX_SECS-1 must bypass skew check regardless of now"
     );
 }
 
-/// timestamp = BOOT_TIME_MAX_SECS is the first value outside the boot-time window.
+/// timestamp = BOOT_TIME_COMPAT_MAX_SECS is the first value outside the
+/// runtime boot-time compatibility window.
 /// is_boot_time = false → skew check IS applied.  Two sub-cases confirm this:
 /// once with now chosen so the skew passes (accepted) and once where it fails.
 #[test]
 fn timestamp_at_boot_threshold_triggers_skew_check() {
     let secret = b"boot_exact_value_test";
-    let ts: u32 = BOOT_TIME_MAX_SECS;
+    let ts: u32 = BOOT_TIME_COMPAT_MAX_SECS;
     let h = make_valid_tls_handshake(secret, ts);
     let secrets = vec![("u".to_string(), secret.to_vec())];
 
     // now = ts + 50 → time_diff = 50, within [-1200, 600] → accepted.
     let now_valid: i64 = ts as i64 + 50;
     assert!(
-        validate_tls_handshake_at_time(&h, &secrets, false, now_valid).is_some(),
+        validate_tls_handshake_at_time_with_boot_cap(
-        "ts=BOOT_TIME_MAX_SECS within skew window must be accepted via skew check"
+            &h,
+            &secrets,
+            false,
+            now_valid,
+            BOOT_TIME_COMPAT_MAX_SECS,
+        )
+        .is_some(),
+        "ts=BOOT_TIME_COMPAT_MAX_SECS within skew window must be accepted via skew check"
     );
 
-    // now = 0 → time_diff = -86_400_000, outside window → rejected.
+    // now = -1 → time_diff = -121 at the 120-second threshold, outside window
-    // If the boot-time bypass were wrongly applied here this would pass.
+    // for TIME_SKEW_MIN=-120. If boot-time bypass were wrongly applied this
+    // would pass.
     assert!(
-        validate_tls_handshake_at_time(&h, &secrets, false, 0).is_none(),
+        validate_tls_handshake_at_time_with_boot_cap(
-        "ts=BOOT_TIME_MAX_SECS far from now must be rejected — no boot-time bypass"
+            &h,
+            &secrets,
+            false,
+            -1,
+            BOOT_TIME_COMPAT_MAX_SECS,
+        )
+        .is_none(),
+        "ts=BOOT_TIME_COMPAT_MAX_SECS far from now must be rejected — no boot-time bypass"
     );
 }
 
@@ -723,7 +740,7 @@ fn replay_window_cap_disables_boot_bypass_for_old_timestamps() {
 #[test]
 fn replay_window_cap_still_allows_small_boot_timestamp() {
     let secret = b"boot_cap_enabled_test";
-    let ts: u32 = 120;
+    let ts: u32 = BOOT_TIME_COMPAT_MAX_SECS.saturating_sub(1);
     let h = make_valid_tls_handshake(secret, ts);
     let secrets = vec![("u".to_string(), secret.to_vec())];
 
@@ -734,6 +751,20 @@ fn replay_window_cap_still_allows_small_boot_timestamp() {
     );
 }
 
+#[test]
+fn large_replay_window_is_hard_capped_for_boot_compatibility() {
+    let secret = b"boot_cap_hard_limit_test";
+    let ts: u32 = BOOT_TIME_COMPAT_MAX_SECS + 1;
+    let h = make_valid_tls_handshake(secret, ts);
+    let secrets = vec![("u".to_string(), secret.to_vec())];
+
+    let result = validate_tls_handshake_with_replay_window(&h, &secrets, false, u64::MAX);
+    assert!(
+        result.is_none(),
+        "very large replay window must not expand boot-time bypass beyond hard compatibility cap"
+    );
+}
+
 #[test]
 fn ignore_time_skew_explicitly_decouples_from_boot_time_cap() {
     let secret = b"ignore_skew_boot_cap_decouple_test";
@@ -743,7 +774,7 @@ fn ignore_time_skew_explicitly_decouples_from_boot_time_cap() {
 
     let cap_zero = validate_tls_handshake_at_time_with_boot_cap(&h, &secrets, true, 0, 0);
     let cap_nonzero =
-        validate_tls_handshake_at_time_with_boot_cap(&h, &secrets, true, 0, BOOT_TIME_MAX_SECS);
+        validate_tls_handshake_at_time_with_boot_cap(&h, &secrets, true, 0, BOOT_TIME_COMPAT_MAX_SECS);
 
     assert!(cap_zero.is_some(), "ignore_time_skew=true must accept valid HMAC");
     assert!(
@@ -1889,6 +1920,228 @@ fn server_hello_new_session_ticket_count_matches_configuration() {
     );
 }
 
+#[test]
+fn server_hello_new_session_ticket_count_is_safely_capped() {
+    let secret = b"ticket_count_cap_test";
+    let client_digest = [0x44u8; TLS_DIGEST_LEN];
+    let session_id = vec![0x54; 32];
+    let rng = crate::crypto::SecureRandom::new();
+
+    let response = build_server_hello(secret, &client_digest, &session_id, 1024, &rng, None, u8::MAX);
+
+    let mut pos = 0usize;
+    let mut app_records = 0usize;
+    while pos + 5 <= response.len() {
+        let rtype = response[pos];
+        let rlen = u16::from_be_bytes([response[pos + 3], response[pos + 4]]) as usize;
+        let next = pos + 5 + rlen;
+        assert!(next <= response.len(), "TLS record must stay inside response bounds");
+        if rtype == TLS_RECORD_APPLICATION {
+            app_records += 1;
+        }
+        pos = next;
+    }
+
+    assert_eq!(
+        app_records,
+        5,
+        "response must cap ticket-like tail records to four plus one main application record"
+    );
+}
+
+#[test]
+fn server_hello_application_data_contains_alpn_marker_when_selected() {
+    let secret = b"alpn_marker_test";
+    let client_digest = [0x55u8; TLS_DIGEST_LEN];
+    let session_id = vec![0xAB; 32];
+    let rng = crate::crypto::SecureRandom::new();
+
+    let response = build_server_hello(
+        secret,
+        &client_digest,
+        &session_id,
+        512,
+        &rng,
+        Some(b"h2".to_vec()),
+        0,
+    );
+
+    let sh_len = u16::from_be_bytes([response[3], response[4]]) as usize;
+    let ccs_pos = 5 + sh_len;
+    let ccs_len = u16::from_be_bytes([response[ccs_pos + 3], response[ccs_pos + 4]]) as usize;
+    let app_pos = ccs_pos + 5 + ccs_len;
+    let app_len = u16::from_be_bytes([response[app_pos + 3], response[app_pos + 4]]) as usize;
+    let app_payload = &response[app_pos + 5..app_pos + 5 + app_len];
+
+    let expected = [0x00u8, 0x10, 0x00, 0x05, 0x00, 0x03, 0x02, b'h', b'2'];
+    assert!(
+        app_payload.windows(expected.len()).any(|window| window == expected),
+        "first application payload must carry ALPN marker for selected protocol"
+    );
+}
+
+#[test]
+fn server_hello_ignores_oversized_alpn_and_still_caps_ticket_tail() {
+    let secret = b"alpn_oversize_ignore_test";
+    let client_digest = [0x56u8; TLS_DIGEST_LEN];
+    let session_id = vec![0xCD; 32];
+    let rng = crate::crypto::SecureRandom::new();
+    let oversized_alpn = vec![b'x'; u8::MAX as usize + 1];
+
+    let response = build_server_hello(
+        secret,
+        &client_digest,
+        &session_id,
+        512,
+        &rng,
+        Some(oversized_alpn),
+        u8::MAX,
+    );
+
+    let mut pos = 0usize;
+    let mut app_records = 0usize;
+    let mut first_app_payload: Option<&[u8]> = None;
+    while pos + 5 <= response.len() {
+        let rtype = response[pos];
+        let rlen = u16::from_be_bytes([response[pos + 3], response[pos + 4]]) as usize;
+        let next = pos + 5 + rlen;
+        assert!(next <= response.len(), "TLS record must stay inside response bounds");
+        if rtype == TLS_RECORD_APPLICATION {
+            app_records += 1;
+            if first_app_payload.is_none() {
+                first_app_payload = Some(&response[pos + 5..next]);
+            }
+        }
+        pos = next;
+    }
+    let marker = [0x00u8, 0x10, 0x00, 0x06, 0x00, 0x04, 0x03, b'x', b'x', b'x', b'x'];
+
+    assert_eq!(
+        app_records, 5,
+        "oversized ALPN must not change the four-ticket cap on tail records"
+    );
+    assert!(
+        !first_app_payload
+            .expect("response must contain an application record")
+            .windows(marker.len())
+            .any(|window| window == marker),
+        "oversized ALPN must be ignored rather than embedded into the first application payload"
+    );
+}
+
+#[test]
+fn server_hello_ignores_oversized_alpn_when_marker_would_not_fit() {
+    let secret = b"alpn_too_large_to_fit_test";
+    let client_digest = [0x57u8; TLS_DIGEST_LEN];
+    let session_id = vec![0xEF; 32];
+    let rng = crate::crypto::SecureRandom::new();
+    let oversized_alpn = vec![0xAB; u8::MAX as usize];
+
+    let response = build_server_hello(
+        secret,
+        &client_digest,
+        &session_id,
+        64,
+        &rng,
+        Some(oversized_alpn),
+        0,
+    );
+
+    let sh_len = u16::from_be_bytes([response[3], response[4]]) as usize;
+    let ccs_pos = 5 + sh_len;
+    let ccs_len = u16::from_be_bytes([response[ccs_pos + 3], response[ccs_pos + 4]]) as usize;
+    let app_pos = ccs_pos + 5 + ccs_len;
+    let app_len = u16::from_be_bytes([response[app_pos + 3], response[app_pos + 4]]) as usize;
+    let app_payload = &response[app_pos + 5..app_pos + 5 + app_len];
+
+    let mut marker_prefix = Vec::new();
+    marker_prefix.extend_from_slice(&0x0010u16.to_be_bytes());
+    marker_prefix.extend_from_slice(&0x0102u16.to_be_bytes());
+    marker_prefix.extend_from_slice(&0x0100u16.to_be_bytes());
+    marker_prefix.push(0xff);
+    marker_prefix.extend_from_slice(&[0xab; 8]);
+    assert!(
+        !app_payload.starts_with(&marker_prefix),
+        "oversized ALPN must not be partially embedded into the ServerHello application record"
+    );
+}
+
+#[test]
+fn server_hello_embeds_full_alpn_marker_when_it_exactly_fits_fake_cert_len() {
+    let secret = b"alpn_exact_fit_test";
+    let client_digest = [0x58u8; TLS_DIGEST_LEN];
+    let session_id = vec![0xA5; 32];
+    let rng = crate::crypto::SecureRandom::new();
+    let proto = vec![b'z'; 57];
+
+    // marker_len = 4 + (2 + (1 + proto_len)) = 7 + proto_len = 64
+    let response = build_server_hello(
+        secret,
+        &client_digest,
+        &session_id,
+        64,
+        &rng,
+        Some(proto.clone()),
+        0,
+    );
+
+    let sh_len = u16::from_be_bytes([response[3], response[4]]) as usize;
+    let ccs_pos = 5 + sh_len;
+    let ccs_len = u16::from_be_bytes([response[ccs_pos + 3], response[ccs_pos + 4]]) as usize;
+    let app_pos = ccs_pos + 5 + ccs_len;
+    let app_len = u16::from_be_bytes([response[app_pos + 3], response[app_pos + 4]]) as usize;
+    let app_payload = &response[app_pos + 5..app_pos + 5 + app_len];
+
+    let mut expected_marker = Vec::new();
+    expected_marker.extend_from_slice(&0x0010u16.to_be_bytes());
+    expected_marker.extend_from_slice(&0x003Cu16.to_be_bytes());
+    expected_marker.extend_from_slice(&0x003Au16.to_be_bytes());
+    expected_marker.push(57u8);
+    expected_marker.extend_from_slice(&proto);
+
+    assert_eq!(app_payload.len(), expected_marker.len());
+    assert_eq!(app_payload, expected_marker.as_slice());
+}
+
+#[test]
+fn server_hello_does_not_embed_partial_alpn_marker_when_one_byte_short() {
+    let secret = b"alpn_one_byte_short_test";
+    let client_digest = [0x59u8; TLS_DIGEST_LEN];
+    let session_id = vec![0xA6; 32];
+    let rng = crate::crypto::SecureRandom::new();
+    let proto = vec![0xAB; 58];
+
+    // marker_len = 65, fake_cert_len = 64 => marker must be fully skipped.
+    let response = build_server_hello(
+        secret,
+        &client_digest,
+        &session_id,
+        64,
+        &rng,
+        Some(proto),
+        0,
+    );
+
+    let sh_len = u16::from_be_bytes([response[3], response[4]]) as usize;
+    let ccs_pos = 5 + sh_len;
+    let ccs_len = u16::from_be_bytes([response[ccs_pos + 3], response[ccs_pos + 4]]) as usize;
+    let app_pos = ccs_pos + 5 + ccs_len;
+    let app_len = u16::from_be_bytes([response[app_pos + 3], response[app_pos + 4]]) as usize;
+    let app_payload = &response[app_pos + 5..app_pos + 5 + app_len];
+
+    let mut marker_prefix = Vec::new();
+    marker_prefix.extend_from_slice(&0x0010u16.to_be_bytes());
+    marker_prefix.extend_from_slice(&0x003Du16.to_be_bytes());
+    marker_prefix.extend_from_slice(&0x003Bu16.to_be_bytes());
+    marker_prefix.push(58u8);
+    marker_prefix.extend_from_slice(&[0xAB; 8]);
+
+    assert!(
+        !app_payload.starts_with(&marker_prefix),
+        "one-byte-short ALPN marker must be skipped entirely, not partially embedded"
+    );
+}
+
 #[test]
 fn exhaustive_tls_minor_version_classification_matches_policy() {
     for minor in 0u8..=u8::MAX {

src/proxy/client.rs

@@ -31,19 +31,16 @@ struct UserConnectionReservation {
     user: String,
     ip: IpAddr,
     active: bool,
-    runtime_handle: Option<tokio::runtime::Handle>,
 }
 
 impl UserConnectionReservation {
     fn new(stats: Arc<Stats>, ip_tracker: Arc<UserIpTracker>, user: String, ip: IpAddr) -> Self {
-        let runtime_handle = tokio::runtime::Handle::try_current().ok();
         Self {
             stats,
             ip_tracker,
             user,
             ip,
             active: true,
-            runtime_handle,
         }
     }
 
@@ -64,29 +61,7 @@ impl Drop for UserConnectionReservation {
         }
         self.active = false;
         self.stats.decrement_user_curr_connects(&self.user);
-
+        self.ip_tracker.enqueue_cleanup(self.user.clone(), self.ip);
-        if let Some(handle) = &self.runtime_handle {
-            let ip_tracker = self.ip_tracker.clone();
-            let user = self.user.clone();
-            let ip = self.ip;
-            let handle = handle.clone();
-            handle.spawn(async move {
-                ip_tracker.remove_ip(&user, ip).await;
-            });
-        } else if let Ok(handle) = tokio::runtime::Handle::try_current() {
-            let ip_tracker = self.ip_tracker.clone();
-            let user = self.user.clone();
-            let ip = self.ip;
-            handle.spawn(async move {
-                ip_tracker.remove_ip(&user, ip).await;
-            });
-        } else {
-            warn!(
-                user = %self.user,
-                ip = %self.ip,
-                "UserConnectionReservation dropped without Tokio runtime; IP reservation cleanup skipped"
-            );
-        }
     }
 }
 

src/proxy/client_security_tests.rs

@@ -42,6 +42,35 @@ where
     CryptoWriter::new(writer, AesCtr::new(&key, iv), 8 * 1024)
 }
 
+#[tokio::test]
+async fn user_connection_reservation_drop_enqueues_cleanup_synchronously() {
+    let ip_tracker = Arc::new(crate::ip_tracker::UserIpTracker::new());
+    let stats = Arc::new(crate::stats::Stats::new());
+    let user = "sync-drop-user".to_string();
+    let ip: std::net::IpAddr = "192.168.1.1".parse().unwrap();
+    
+    ip_tracker.set_user_limit(&user, 1).await;
+    ip_tracker.check_and_add(&user, ip).await.unwrap();
+    stats.increment_user_curr_connects(&user);
+    
+    assert_eq!(ip_tracker.get_active_ip_count(&user).await, 1);
+    assert_eq!(stats.get_user_curr_connects(&user), 1);
+    
+    let reservation = UserConnectionReservation::new(stats.clone(), ip_tracker.clone(), user.clone(), ip);
+    
+    // Drop the reservation synchronously without any tokio::spawn/await yielding!
+    drop(reservation);
+    
+    // The IP is now inside the cleanup_queue, check that the queue has length 1
+    let queue_len = ip_tracker.cleanup_queue.lock().unwrap().len();
+    assert_eq!(queue_len, 1, "Reservation drop must push directly to synchronized IP queue");
+    
+    assert_eq!(stats.get_user_curr_connects(&user), 0, "Stats must decrement immediately");
+    
+    ip_tracker.drain_cleanup_queue().await;
+    assert_eq!(ip_tracker.get_active_ip_count(&user).await, 0);
+}
+
 #[tokio::test]
 async fn relay_task_abort_releases_user_gate_and_ip_reservation() {
     let tg_listener = TcpListener::bind("127.0.0.1:0").await.unwrap();

src/proxy/direct_relay.rs

@@ -132,7 +132,11 @@ fn open_unknown_dc_log_append(path: &Path) -> std::io::Result<std::fs::File> {
     }
     #[cfg(not(unix))]
     {
-        OpenOptions::new().create(true).append(true).open(path)
+        let _ = path;
+        Err(std::io::Error::new(
+            std::io::ErrorKind::PermissionDenied,
+            "unknown_dc_file_log_enabled requires unix O_NOFOLLOW support",
+        ))
     }
 }
 
@@ -204,6 +208,7 @@ where
         config.general.direct_relay_copy_buf_s2c_bytes,
         user,
         Arc::clone(&stats),
+        config.access.user_data_quota.get(user).copied(),
         buffer_pool,
     );
     tokio::pin!(relay_result);

src/proxy/handshake.rs

@@ -241,7 +241,26 @@ fn auth_probe_record_failure_with_state(
             rounds += 1;
             if rounds > 8 {
                 auth_probe_note_saturation(now);
-                return;
+                let mut eviction_candidate: Option<(IpAddr, u32, Instant)> = None;
+                for entry in state.iter().take(AUTH_PROBE_PRUNE_SCAN_LIMIT) {
+                    let key = *entry.key();
+                    let fail_streak = entry.value().fail_streak;
+                    let last_seen = entry.value().last_seen;
+                    match eviction_candidate {
+                        Some((_, current_fail, current_seen))
+                            if fail_streak > current_fail
+                                || (fail_streak == current_fail && last_seen >= current_seen) =>
+                        {
+                        }
+                        _ => eviction_candidate = Some((key, fail_streak, last_seen)),
+                    }
+                }
+
+                let Some((evict_key, _, _)) = eviction_candidate else {
+                    return;
+                };
+                state.remove(&evict_key);
+                break;
             }
 
             let mut stale_keys = Vec::new();
@@ -518,6 +537,7 @@ pub struct HandshakeSuccess {
     /// Client address
     pub peer: SocketAddr,
     /// Whether TLS was used
+    
     pub is_tls: bool,
 }
 
@@ -716,7 +736,11 @@ where
     R: AsyncRead + Unpin + Send,
     W: AsyncWrite + Unpin + Send,
 {
-    trace!(peer = %peer, handshake = ?hex::encode(handshake), "MTProto handshake bytes");
+    trace!(
+        peer = %peer,
+        handshake_head = %hex::encode(&handshake[..8]),
+        "MTProto handshake prefix"
+    );
 
     let throttle_now = Instant::now();
     if auth_probe_should_apply_preauth_throttle(peer.ip(), throttle_now) {
@@ -916,6 +940,7 @@ pub fn encrypt_tg_nonce_with_ciphers(nonce: &[u8; HANDSHAKE_LEN]) -> (Vec<u8>, A
 }
 
 /// Encrypt nonce for sending to Telegram (legacy function for compatibility)
+
 pub fn encrypt_tg_nonce(nonce: &[u8; HANDSHAKE_LEN]) -> Vec<u8> {
     let (encrypted, _, _) = encrypt_tg_nonce_with_ciphers(nonce);
     encrypted

src/proxy/handshake_security_tests.rs

@@ -1584,6 +1584,47 @@ fn stress_auth_probe_full_map_churn_keeps_bound_and_tracks_newcomers() {
     }
 }
 
+#[test]
+fn auth_probe_over_cap_churn_still_tracks_newcomer_after_round_limit() {
+    let _guard = auth_probe_test_lock()
+        .lock()
+        .unwrap_or_else(|poisoned| poisoned.into_inner());
+    clear_auth_probe_state_for_testing();
+
+    let state = DashMap::new();
+    let now = Instant::now();
+    let initial = AUTH_PROBE_TRACK_MAX_ENTRIES + 32;
+
+    for idx in 0..initial {
+        let ip = IpAddr::V4(Ipv4Addr::new(
+            10,
+            6,
+            ((idx >> 8) & 0xff) as u8,
+            (idx & 0xff) as u8,
+        ));
+        state.insert(
+            ip,
+            AuthProbeState {
+                fail_streak: 1,
+                blocked_until: now,
+                last_seen: now + Duration::from_millis((idx % 1024) as u64),
+            },
+        );
+    }
+
+    let newcomer = IpAddr::V4(Ipv4Addr::new(203, 0, 114, 77));
+    auth_probe_record_failure_with_state(&state, newcomer, now + Duration::from_secs(1));
+
+    assert!(
+        state.get(&newcomer).is_some(),
+        "new probe source must still be tracked even when map starts above hard cap"
+    );
+    assert!(
+        state.len() < initial + 1,
+        "round-limited eviction path must still reclaim capacity under over-cap churn"
+    );
+}
+
 #[test]
 fn auth_probe_capacity_prefers_evicting_low_fail_streak_entries_first() {
     let _guard = auth_probe_test_lock()

src/proxy/middle_relay.rs

@@ -2,15 +2,13 @@ use std::collections::hash_map::RandomState;
 use std::hash::BuildHasher;
 use std::hash::{Hash, Hasher};
 use std::net::{IpAddr, SocketAddr};
-use std::sync::atomic::{AtomicU64, Ordering};
+use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
-use std::sync::{Arc, OnceLock};
+use std::sync::{Arc, Mutex, OnceLock};
 use std::time::{Duration, Instant};
-#[cfg(test)]
-use std::sync::Mutex;
 
 use dashmap::DashMap;
 use tokio::io::{AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt};
-use tokio::sync::{mpsc, oneshot, watch};
+use tokio::sync::{mpsc, oneshot, watch, Mutex as AsyncMutex};
 use tokio::time::timeout;
 use tracing::{debug, trace, warn};
 
@@ -35,14 +33,22 @@ enum C2MeCommand {
 const DESYNC_DEDUP_WINDOW: Duration = Duration::from_secs(60);
 const DESYNC_DEDUP_MAX_ENTRIES: usize = 65_536;
 const DESYNC_DEDUP_PRUNE_SCAN_LIMIT: usize = 1024;
+const DESYNC_FULL_CACHE_EMIT_MIN_INTERVAL: Duration = Duration::from_millis(1000);
 const DESYNC_ERROR_CLASS: &str = "frame_too_large_crypto_desync";
 const C2ME_CHANNEL_CAPACITY_FALLBACK: usize = 128;
 const C2ME_SOFT_PRESSURE_MIN_FREE_SLOTS: usize = 64;
 const C2ME_SENDER_FAIRNESS_BUDGET: usize = 32;
+#[cfg(test)]
+const C2ME_SEND_TIMEOUT: Duration = Duration::from_millis(50);
+#[cfg(not(test))]
+const C2ME_SEND_TIMEOUT: Duration = Duration::from_secs(5);
 const ME_D2C_FLUSH_BATCH_MAX_FRAMES_MIN: usize = 1;
 const ME_D2C_FLUSH_BATCH_MAX_BYTES_MIN: usize = 4096;
 static DESYNC_DEDUP: OnceLock<DashMap<u64, Instant>> = OnceLock::new();
 static DESYNC_HASHER: OnceLock<RandomState> = OnceLock::new();
+static DESYNC_FULL_CACHE_LAST_EMIT_AT: OnceLock<Mutex<Option<Instant>>> = OnceLock::new();
+static DESYNC_DEDUP_EVER_SATURATED: OnceLock<AtomicBool> = OnceLock::new();
+static QUOTA_USER_LOCKS: OnceLock<DashMap<String, Arc<AsyncMutex<()>>>> = OnceLock::new();
 
 struct RelayForensicsState {
     trace_id: u64,
@@ -98,6 +104,11 @@ fn should_emit_full_desync(key: u64, all_full: bool, now: Instant) -> bool {
     }
 
     let dedup = DESYNC_DEDUP.get_or_init(DashMap::new);
+    let saturated_before = dedup.len() >= DESYNC_DEDUP_MAX_ENTRIES;
+    let ever_saturated = DESYNC_DEDUP_EVER_SATURATED.get_or_init(|| AtomicBool::new(false));
+    if saturated_before {
+        ever_saturated.store(true, Ordering::Relaxed);
+    }
 
     if let Some(mut seen_at) = dedup.get_mut(&key) {
         if now.duration_since(*seen_at) >= DESYNC_DEDUP_WINDOW {
@@ -132,12 +143,52 @@ fn should_emit_full_desync(key: u64, all_full: bool, now: Instant) -> bool {
             };
             dedup.remove(&evict_key);
             dedup.insert(key, now);
-            return false;
+            return should_emit_full_desync_full_cache(now);
         }
     }
 
     dedup.insert(key, now);
-    true
+    let saturated_after = dedup.len() >= DESYNC_DEDUP_MAX_ENTRIES;
+    // Preserve the first sequential insert that reaches capacity as a normal
+    // emit, while still gating concurrent newcomer churn after the cache has
+    // ever been observed at saturation.
+    let was_ever_saturated = if saturated_after {
+        ever_saturated.swap(true, Ordering::Relaxed)
+    } else {
+        ever_saturated.load(Ordering::Relaxed)
+    };
+
+    if saturated_before || (saturated_after && was_ever_saturated) {
+        should_emit_full_desync_full_cache(now)
+    } else {
+        true
+    }
+}
+
+fn should_emit_full_desync_full_cache(now: Instant) -> bool {
+    let gate = DESYNC_FULL_CACHE_LAST_EMIT_AT.get_or_init(|| Mutex::new(None));
+    let Ok(mut last_emit_at) = gate.lock() else {
+        return false;
+    };
+
+    match *last_emit_at {
+        None => {
+            *last_emit_at = Some(now);
+            true
+        }
+        Some(last) => {
+            let Some(elapsed) = now.checked_duration_since(last) else {
+                *last_emit_at = Some(now);
+                return true;
+            };
+            if elapsed >= DESYNC_FULL_CACHE_EMIT_MIN_INTERVAL {
+                *last_emit_at = Some(now);
+                true
+            } else {
+                false
+            }
+        }
+    }
 }
 
 #[cfg(test)]
@@ -145,6 +196,21 @@ fn clear_desync_dedup_for_testing() {
     if let Some(dedup) = DESYNC_DEDUP.get() {
         dedup.clear();
     }
+    if let Some(ever_saturated) = DESYNC_DEDUP_EVER_SATURATED.get() {
+        ever_saturated.store(false, Ordering::Relaxed);
+    }
+    if let Some(last_emit_at) = DESYNC_FULL_CACHE_LAST_EMIT_AT.get() {
+        match last_emit_at.lock() {
+            Ok(mut guard) => {
+                *guard = None;
+            }
+            Err(poisoned) => {
+                let mut guard = poisoned.into_inner();
+                *guard = None;
+                last_emit_at.clear_poison();
+            }
+        }
+    }
 }
 
 #[cfg(test)]
@@ -248,6 +314,38 @@ fn should_yield_c2me_sender(sent_since_yield: usize, has_backlog: bool) -> bool
     has_backlog && sent_since_yield >= C2ME_SENDER_FAIRNESS_BUDGET
 }
 
+fn quota_exceeded_for_user(stats: &Stats, user: &str, quota_limit: Option<u64>) -> bool {
+    quota_limit.is_some_and(|quota| stats.get_user_total_octets(user) >= quota)
+}
+
+fn quota_would_be_exceeded_for_user(
+    stats: &Stats,
+    user: &str,
+    quota_limit: Option<u64>,
+    bytes: u64,
+) -> bool {
+    quota_limit.is_some_and(|quota| {
+        let used = stats.get_user_total_octets(user);
+        used >= quota || bytes > quota.saturating_sub(used)
+    })
+}
+
+fn quota_user_lock(user: &str) -> Arc<AsyncMutex<()>> {
+    let locks = QUOTA_USER_LOCKS.get_or_init(DashMap::new);
+    if let Some(existing) = locks.get(user) {
+        return Arc::clone(existing.value());
+    }
+
+    let created = Arc::new(AsyncMutex::new(()));
+    match locks.entry(user.to_string()) {
+        dashmap::mapref::entry::Entry::Occupied(entry) => Arc::clone(entry.get()),
+        dashmap::mapref::entry::Entry::Vacant(entry) => {
+            entry.insert(Arc::clone(&created));
+            created
+        }
+    }
+}
+
 async fn enqueue_c2me_command(
     tx: &mpsc::Sender<C2MeCommand>,
     cmd: C2MeCommand,
@@ -260,7 +358,14 @@ async fn enqueue_c2me_command(
             if tx.capacity() <= C2ME_SOFT_PRESSURE_MIN_FREE_SLOTS {
                 tokio::task::yield_now().await;
             }
-            tx.send(cmd).await
+            match timeout(C2ME_SEND_TIMEOUT, tx.reserve()).await {
+                Ok(Ok(permit)) => {
+                    permit.send(cmd);
+                    Ok(())
+                }
+                Ok(Err(_)) => Err(mpsc::error::SendError(cmd)),
+                Err(_) => Err(mpsc::error::SendError(cmd)),
+            }
         }
     }
 }
@@ -284,6 +389,7 @@ where
     W: AsyncWrite + Unpin + Send + 'static,
 {
     let user = success.user.clone();
+    let quota_limit = config.access.user_data_quota.get(&user).copied();
     let peer = success.peer;
     let proto_tag = success.proto_tag;
     let pool_generation = me_pool.current_generation();
@@ -432,6 +538,7 @@ where
                         &mut frame_buf,
                         stats_clone.as_ref(),
                         &user_clone,
+                        quota_limit,
                         bytes_me2c_clone.as_ref(),
                         conn_id,
                         d2c_flush_policy.ack_flush_immediate,
@@ -464,6 +571,7 @@ where
                             &mut frame_buf,
                             stats_clone.as_ref(),
                             &user_clone,
+                            quota_limit,
                             bytes_me2c_clone.as_ref(),
                             conn_id,
                             d2c_flush_policy.ack_flush_immediate,
@@ -496,6 +604,7 @@ where
                                     &mut frame_buf,
                                     stats_clone.as_ref(),
                                     &user_clone,
+                                    quota_limit,
                                     bytes_me2c_clone.as_ref(),
                                     conn_id,
                                     d2c_flush_policy.ack_flush_immediate,
@@ -528,6 +637,7 @@ where
                                         &mut frame_buf,
                                         stats_clone.as_ref(),
                                         &user_clone,
+                                        quota_limit,
                                         bytes_me2c_clone.as_ref(),
                                         conn_id,
                                         d2c_flush_policy.ack_flush_immediate,
@@ -609,7 +719,19 @@ where
                         forensics.bytes_c2me = forensics
                             .bytes_c2me
                             .saturating_add(payload.len() as u64);
-                        stats.add_user_octets_from(&user, payload.len() as u64);
+                        if let Some(limit) = quota_limit {
+                            let quota_lock = quota_user_lock(&user);
+                            let _quota_guard = quota_lock.lock().await;
+                            stats.add_user_octets_from(&user, payload.len() as u64);
+                            if quota_exceeded_for_user(stats.as_ref(), &user, Some(limit)) {
+                                main_result = Err(ProxyError::DataQuotaExceeded {
+                                    user: user.clone(),
+                                });
+                                break;
+                            }
+                        } else {
+                            stats.add_user_octets_from(&user, payload.len() as u64);
+                        }
                         let mut flags = proto_flags;
                         if quickack {
                             flags |= RPC_FLAG_QUICKACK;
@@ -833,6 +955,7 @@ async fn process_me_writer_response<W>(
     frame_buf: &mut Vec<u8>,
     stats: &Stats,
     user: &str,
+    quota_limit: Option<u64>,
     bytes_me2c: &AtomicU64,
     conn_id: u64,
     ack_flush_immediate: bool,
@@ -848,17 +971,47 @@ where
             } else {
                 trace!(conn_id, bytes = data.len(), flags, "ME->C data");
             }
-            bytes_me2c.fetch_add(data.len() as u64, Ordering::Relaxed);
+            let data_len = data.len() as u64;
-            stats.add_user_octets_to(user, data.len() as u64);
+            if let Some(limit) = quota_limit {
-            write_client_payload(
+                let quota_lock = quota_user_lock(user);
-                client_writer,
+                let _quota_guard = quota_lock.lock().await;
-                proto_tag,
+                if quota_would_be_exceeded_for_user(stats, user, Some(limit), data_len) {
-                flags,
+                    return Err(ProxyError::DataQuotaExceeded {
-                &data,
+                        user: user.to_string(),
-                rng,
+                    });
-                frame_buf,
+                }
-            )
+                write_client_payload(
-            .await?;
+                    client_writer,
+                    proto_tag,
+                    flags,
+                    &data,
+                    rng,
+                    frame_buf,
+                )
+                .await?;
+
+                bytes_me2c.fetch_add(data.len() as u64, Ordering::Relaxed);
+                stats.add_user_octets_to(user, data.len() as u64);
+
+                if quota_exceeded_for_user(stats, user, Some(limit)) {
+                    return Err(ProxyError::DataQuotaExceeded {
+                        user: user.to_string(),
+                    });
+                }
+            } else {
+                write_client_payload(
+                    client_writer,
+                    proto_tag,
+                    flags,
+                    &data,
+                    rng,
+                    frame_buf,
+                )
+                .await?;
+
+                bytes_me2c.fetch_add(data.len() as u64, Ordering::Relaxed);
+                stats.add_user_octets_to(user, data.len() as u64);
+            }
 
             Ok(MeWriterResponseOutcome::Continue {
                 frames: 1,

src/proxy/middle_relay_security_tests.rs

@@ -13,8 +13,9 @@ use rand::{Rng, SeedableRng};
 use std::collections::{HashMap, HashSet};
 use std::net::SocketAddr;
 use std::sync::Arc;
-use std::sync::atomic::AtomicU64;
+use std::sync::atomic::{AtomicU64, AtomicUsize, Ordering};
-use tokio::io::AsyncWriteExt;
+use std::thread;
+use tokio::io::AsyncReadExt;
 use tokio::io::duplex;
 use tokio::time::{Duration as TokioDuration, timeout};
 
@@ -176,6 +177,36 @@ async fn enqueue_c2me_command_full_then_closed_recycles_waiting_payload() {
     );
 }
 
+#[tokio::test]
+async fn enqueue_c2me_command_full_queue_times_out_without_receiver_progress() {
+    let (tx, _rx) = mpsc::channel::<C2MeCommand>(1);
+    tx.send(C2MeCommand::Data {
+        payload: make_pooled_payload(&[1]),
+        flags: 0,
+    })
+    .await
+    .unwrap();
+
+    let started = Instant::now();
+    let result = enqueue_c2me_command(
+        &tx,
+        C2MeCommand::Data {
+            payload: make_pooled_payload(&[2, 2]),
+            flags: 1,
+        },
+    )
+    .await;
+
+    assert!(
+        result.is_err(),
+        "enqueue must fail when queue stays full beyond bounded timeout"
+    );
+    assert!(
+        started.elapsed() < TokioDuration::from_millis(400),
+        "full-queue timeout must resolve promptly"
+    );
+}
+
 #[test]
 fn desync_dedup_cache_is_bounded() {
     let _guard = desync_dedup_test_lock()
@@ -192,12 +223,12 @@ fn desync_dedup_cache_is_bounded() {
     }
 
     assert!(
-        !should_emit_full_desync(u64::MAX, false, now),
+        should_emit_full_desync(u64::MAX, false, now),
-        "new key above cap must remain suppressed to avoid log amplification"
+        "new key above cap must emit once after bounded eviction for forensic visibility"
     );
 
     assert!(
-        !should_emit_full_desync(7, false, now),
+        !should_emit_full_desync(u64::MAX, false, now),
         "already tracked key inside dedup window must stay suppressed"
     );
 }
@@ -215,10 +246,18 @@ fn desync_dedup_full_cache_churn_stays_suppressed() {
     }
 
     for offset in 0..2048u64 {
-        assert!(
+        let emitted = should_emit_full_desync(u64::MAX - offset, false, now);
-            !should_emit_full_desync(u64::MAX - offset, false, now),
+        if offset == 0 {
-            "fresh full-cache churn must remain suppressed under pressure"
+            assert!(
-        );
+                emitted,
+                "first full-cache newcomer should emit for forensic visibility"
+            );
+        } else {
+            assert!(
+                !emitted,
+                "full-cache newcomer churn inside emit interval must stay suppressed"
+            );
+        }
     }
 }
 
@@ -296,18 +335,20 @@ fn stress_desync_dedup_churn_keeps_cache_hard_bounded() {
     let now = Instant::now();
     let total = DESYNC_DEDUP_MAX_ENTRIES + 8192;
 
+    let mut emitted_count = 0usize;
     for key in 0..total as u64 {
         let emitted = should_emit_full_desync(key, false, now);
-        if key < DESYNC_DEDUP_MAX_ENTRIES as u64 {
+        if emitted {
-            assert!(emitted, "keys below cap must be admitted initially");
+            emitted_count += 1;
-        } else {
-            assert!(
-                !emitted,
-                "new keys above cap must stay suppressed under sustained churn"
-            );
         }
     }
 
+    assert_eq!(
+        emitted_count,
+        DESYNC_DEDUP_MAX_ENTRIES + 1,
+        "after capacity is reached, same-tick newcomer churn must be rate-limited"
+    );
+
     let len = DESYNC_DEDUP
         .get()
         .expect("dedup cache must be initialized by stress run")
@@ -318,6 +359,282 @@ fn stress_desync_dedup_churn_keeps_cache_hard_bounded() {
     );
 }
 
+#[test]
+fn full_cache_newcomer_emission_is_rate_limited_but_periodic() {
+    let _guard = desync_dedup_test_lock()
+        .lock()
+        .expect("desync dedup test lock must be available");
+    clear_desync_dedup_for_testing();
+
+    let dedup = DESYNC_DEDUP.get_or_init(DashMap::new);
+    let base_now = Instant::now();
+
+    for key in 0..DESYNC_DEDUP_MAX_ENTRIES as u64 {
+        dedup.insert(key, base_now - TokioDuration::from_millis(10));
+    }
+
+    // Same-tick newcomer storm: only the first should emit full forensic record.
+    let mut burst_emits = 0usize;
+    for i in 0..1024u64 {
+        if should_emit_full_desync(10_000_000 + i, false, base_now) {
+            burst_emits += 1;
+        }
+    }
+    assert_eq!(
+        burst_emits, 1,
+        "full-cache newcomer burst must be bounded to a single full emit per interval"
+    );
+
+    // After each interval elapses, one newcomer may emit again.
+    for step in 1..=6u64 {
+        let t = base_now + DESYNC_FULL_CACHE_EMIT_MIN_INTERVAL * step as u32;
+        assert!(
+            should_emit_full_desync(20_000_000 + step, false, t),
+            "full-cache newcomer should re-emit once interval has elapsed"
+        );
+        assert!(
+            !should_emit_full_desync(30_000_000 + step, false, t),
+            "additional newcomers in the same interval tick must remain suppressed"
+        );
+    }
+}
+
+#[test]
+fn full_cache_mode_override_emits_every_event() {
+    let _guard = desync_dedup_test_lock()
+        .lock()
+        .expect("desync dedup test lock must be available");
+    clear_desync_dedup_for_testing();
+
+    let now = Instant::now();
+    for i in 0..10_000u64 {
+        assert!(
+            should_emit_full_desync(100_000_000 + i, true, now),
+            "desync_all_full override must bypass dedup and rate-limit suppression"
+        );
+    }
+}
+
+#[test]
+fn report_desync_stats_follow_rate_limited_full_cache_policy() {
+    let _guard = desync_dedup_test_lock()
+        .lock()
+        .expect("desync dedup test lock must be available");
+    clear_desync_dedup_for_testing();
+
+    let dedup = DESYNC_DEDUP.get_or_init(DashMap::new);
+    let base_now = Instant::now();
+    for key in 0..DESYNC_DEDUP_MAX_ENTRIES as u64 {
+        dedup.insert(key, base_now - TokioDuration::from_millis(10));
+    }
+
+    let stats = Stats::new();
+    let mut state = make_forensics_state();
+    state.started_at = base_now;
+
+    for i in 0..128u64 {
+        state.peer_hash = 0xABC0_0000_0000_0000u64 ^ i;
+        let _ = report_desync_frame_too_large(
+            &state,
+            ProtoTag::Secure,
+            3,
+            1024,
+            4096,
+            Some([0x16, 0x03, 0x03, 0x00]),
+            &stats,
+        );
+    }
+
+    assert_eq!(
+        stats.get_desync_total(),
+        128,
+        "every detected desync must increment total counter"
+    );
+    assert_eq!(
+        stats.get_desync_full_logged(),
+        1,
+        "same-interval full-cache newcomer storm must allow only one full forensic emit"
+    );
+    assert_eq!(
+        stats.get_desync_suppressed(),
+        127,
+        "remaining same-interval full-cache newcomer events must be suppressed"
+    );
+
+    // After one full interval in real wall clock, a newcomer should emit again.
+    thread::sleep(DESYNC_FULL_CACHE_EMIT_MIN_INTERVAL + TokioDuration::from_millis(20));
+    state.peer_hash = 0xDEAD_BEEF_DEAD_BEEFu64;
+    let _ = report_desync_frame_too_large(
+        &state,
+        ProtoTag::Secure,
+        4,
+        1024,
+        4097,
+        Some([0x16, 0x03, 0x03, 0x01]),
+        &stats,
+    );
+
+    assert_eq!(
+        stats.get_desync_full_logged(),
+        2,
+        "full forensic emission must recover after rate-limit interval"
+    );
+}
+
+#[test]
+fn concurrent_full_cache_newcomer_storm_is_single_emit_per_interval() {
+    let _guard = desync_dedup_test_lock()
+        .lock()
+        .expect("desync dedup test lock must be available");
+    clear_desync_dedup_for_testing();
+
+    let dedup = DESYNC_DEDUP.get_or_init(DashMap::new);
+    let base_now = Instant::now();
+    for key in 0..DESYNC_DEDUP_MAX_ENTRIES as u64 {
+        dedup.insert(key, base_now - TokioDuration::from_millis(10));
+    }
+
+    let emits = Arc::new(AtomicUsize::new(0));
+    let mut workers = Vec::new();
+    for worker_id in 0..32u64 {
+        let emits = Arc::clone(&emits);
+        workers.push(thread::spawn(move || {
+            for i in 0..512u64 {
+                let key = 0x7000_0000_0000_0000u64 ^ (worker_id << 20) ^ i;
+                if should_emit_full_desync(key, false, base_now) {
+                    emits.fetch_add(1, Ordering::Relaxed);
+                }
+            }
+        }));
+    }
+
+    for worker in workers {
+        worker.join().expect("worker thread must not panic");
+    }
+
+    assert_eq!(
+        emits.load(Ordering::Relaxed),
+        1,
+        "concurrent same-interval full-cache storm must allow only one full forensic emit"
+    );
+}
+
+#[test]
+fn light_fuzz_full_cache_rate_limit_oracle_matches_model() {
+    let _guard = desync_dedup_test_lock()
+        .lock()
+        .expect("desync dedup test lock must be available");
+    clear_desync_dedup_for_testing();
+
+    let dedup = DESYNC_DEDUP.get_or_init(DashMap::new);
+    let base_now = Instant::now();
+    for key in 0..DESYNC_DEDUP_MAX_ENTRIES as u64 {
+        dedup.insert(key, base_now - TokioDuration::from_millis(10));
+    }
+
+    let mut rng = StdRng::seed_from_u64(0xD15EA5E5_F00DBAAD);
+    let mut model_last_emit: Option<Instant> = None;
+
+    for i in 0..4096u64 {
+        let jitter_ms: u64 = rng.random_range(0..=3000);
+        let t = base_now + TokioDuration::from_millis(jitter_ms);
+        let key = 0x55AA_0000_0000_0000u64 ^ i ^ rng.random::<u64>();
+        let actual = should_emit_full_desync(key, false, t);
+
+        let expected = match model_last_emit {
+            None => {
+                model_last_emit = Some(t);
+                true
+            }
+            Some(last) => {
+                match t.checked_duration_since(last) {
+                    Some(elapsed) if elapsed >= DESYNC_FULL_CACHE_EMIT_MIN_INTERVAL => {
+                        model_last_emit = Some(t);
+                        true
+                    }
+                    Some(_) => false,
+                    None => {
+                        // Match production fail-open behavior for non-monotonic synthetic input.
+                        model_last_emit = Some(t);
+                        true
+                    }
+                }
+            }
+        };
+
+        assert_eq!(
+            actual, expected,
+            "full-cache rate-limit gate diverged from reference model under light fuzz"
+        );
+    }
+}
+
+#[test]
+fn full_cache_gate_lock_poison_is_fail_closed_without_panic() {
+    let _guard = desync_dedup_test_lock()
+        .lock()
+        .expect("desync dedup test lock must be available");
+    clear_desync_dedup_for_testing();
+
+    let dedup = DESYNC_DEDUP.get_or_init(DashMap::new);
+    let base_now = Instant::now();
+    for key in 0..DESYNC_DEDUP_MAX_ENTRIES as u64 {
+        dedup.insert(key, base_now - TokioDuration::from_millis(10));
+    }
+
+    // Poison the full-cache gate lock intentionally.
+    let gate = DESYNC_FULL_CACHE_LAST_EMIT_AT.get_or_init(|| Mutex::new(None));
+    let _ = std::panic::catch_unwind(|| {
+        let _lock = gate.lock().expect("gate lock must be lockable before poison");
+        panic!("intentional gate poison for fail-closed regression");
+    });
+
+    let emitted = should_emit_full_desync(0xFACE_0000_0000_0001, false, base_now);
+    assert!(
+        !emitted,
+        "poisoned full-cache gate must fail-closed (suppress) instead of panic or fail-open"
+    );
+    assert!(
+        dedup.len() <= DESYNC_DEDUP_MAX_ENTRIES,
+        "dedup cache must remain bounded even when gate lock is poisoned"
+    );
+}
+
+#[test]
+fn full_cache_non_monotonic_time_emits_and_resets_gate_safely() {
+    let _guard = desync_dedup_test_lock()
+        .lock()
+        .expect("desync dedup test lock must be available");
+    clear_desync_dedup_for_testing();
+
+    let dedup = DESYNC_DEDUP.get_or_init(DashMap::new);
+    let base_now = Instant::now();
+    for key in 0..DESYNC_DEDUP_MAX_ENTRIES as u64 {
+        dedup.insert(key, base_now - TokioDuration::from_millis(10));
+    }
+
+    // First event seeds the gate.
+    assert!(should_emit_full_desync(
+        0xABCD_0000_0000_0001,
+        false,
+        base_now + TokioDuration::from_millis(900)
+    ));
+
+    // Synthetic earlier timestamp must not panic; it should fail-open and reset gate.
+    assert!(should_emit_full_desync(
+        0xABCD_0000_0000_0002,
+        false,
+        base_now + TokioDuration::from_millis(100)
+    ));
+
+    // Same instant again remains suppressed after reset.
+    assert!(!should_emit_full_desync(
+        0xABCD_0000_0000_0003,
+        false,
+        base_now + TokioDuration::from_millis(100)
+    ));
+}
+
 #[test]
 fn desync_dedup_full_cache_inserts_new_key_with_bounded_single_key_churn() {
     let _guard = desync_dedup_test_lock()
@@ -338,8 +655,8 @@ fn desync_dedup_full_cache_inserts_new_key_with_bounded_single_key_churn() {
     let newcomer_key = u64::MAX;
     let emitted = should_emit_full_desync(newcomer_key, false, base_now);
     assert!(
-        !emitted,
+        emitted,
-        "new entry under full fresh cache must stay suppressed"
+        "new entry under full fresh cache must emit after bounded eviction"
     );
     assert!(
         dedup.get(&newcomer_key).is_some(),
@@ -406,6 +723,24 @@ fn light_fuzz_desync_dedup_temporal_gate_behavior_is_stable() {
     panic!("expected at least one post-window sample to re-emit forensic record");
 }
 
+#[test]
+#[ignore = "Tracking for M-04: Verify should_emit_full_desync returns true on first occurrence and false on duplicate within window"]
+fn should_emit_full_desync_filters_duplicates() {
+    unimplemented!("Stub for M-04");
+}
+
+#[test]
+#[ignore = "Tracking for M-04: Verify desync dedup eviction behaves correctly under map-full condition"]
+fn desync_dedup_eviction_under_map_full_condition() {
+    unimplemented!("Stub for M-04");
+}
+
+#[tokio::test]
+#[ignore = "Tracking for M-05: Verify C2ME channel full path yields then sends under backpressure"]
+async fn c2me_channel_full_path_yields_then_sends() {
+    unimplemented!("Stub for M-05");
+}
+
 fn make_forensics_state() -> RelayForensicsState {
     RelayForensicsState {
         trace_id: 1,
@@ -974,6 +1309,7 @@ async fn process_me_writer_response_ack_obeys_flush_policy() {
         &mut frame_buf,
         &stats,
         "user",
+        None,
         &bytes_me2c,
         77,
         true,
@@ -999,6 +1335,7 @@ async fn process_me_writer_response_ack_obeys_flush_policy() {
         &mut frame_buf,
         &stats,
         "user",
+        None,
         &bytes_me2c,
         77,
         false,
@@ -1038,6 +1375,7 @@ async fn process_me_writer_response_data_updates_byte_accounting() {
         &mut frame_buf,
         &stats,
         "user",
+        None,
         &bytes_me2c,
         88,
         false,
@@ -1061,6 +1399,162 @@ async fn process_me_writer_response_data_updates_byte_accounting() {
     );
 }
 
+#[tokio::test]
+async fn process_me_writer_response_data_enforces_live_user_quota() {
+    let (writer_side, mut reader_side) = duplex(1024);
+    let mut writer = make_crypto_writer(writer_side);
+    let rng = SecureRandom::new();
+    let mut frame_buf = Vec::new();
+    let stats = Stats::new();
+    let bytes_me2c = AtomicU64::new(0);
+
+    stats.add_user_octets_from("quota-user", 10);
+
+    let result = process_me_writer_response(
+        MeResponse::Data {
+            flags: 0,
+            data: Bytes::from(vec![1u8, 2, 3, 4]),
+        },
+        &mut writer,
+        ProtoTag::Intermediate,
+        &rng,
+        &mut frame_buf,
+        &stats,
+        "quota-user",
+        Some(12),
+        &bytes_me2c,
+        89,
+        false,
+        false,
+    )
+    .await;
+
+    assert!(
+        matches!(result, Err(ProxyError::DataQuotaExceeded { user }) if user == "quota-user"),
+        "ME->client runtime path must terminate when live user quota is crossed"
+    );
+
+    let mut raw = [0u8; 1];
+    assert!(
+        timeout(TokioDuration::from_millis(100), reader_side.read(&mut raw))
+            .await
+            .is_err(),
+        "quota exhaustion must not write any ciphertext to the client stream"
+    );
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn process_me_writer_response_concurrent_same_user_quota_does_not_overshoot_limit() {
+    let stats = Stats::new();
+    let bytes_me2c = AtomicU64::new(0);
+    let user = "quota-race-user";
+
+    let (writer_side_a, _reader_side_a) = duplex(1024);
+    let (writer_side_b, _reader_side_b) = duplex(1024);
+    let mut writer_a = make_crypto_writer(writer_side_a);
+    let mut writer_b = make_crypto_writer(writer_side_b);
+    let mut frame_buf_a = Vec::new();
+    let mut frame_buf_b = Vec::new();
+    let rng_a = SecureRandom::new();
+    let rng_b = SecureRandom::new();
+
+    let fut_a = process_me_writer_response(
+        MeResponse::Data {
+            flags: 0,
+            data: Bytes::from_static(&[0x11]),
+        },
+        &mut writer_a,
+        ProtoTag::Intermediate,
+        &rng_a,
+        &mut frame_buf_a,
+        &stats,
+        user,
+        Some(1),
+        &bytes_me2c,
+        91,
+        false,
+        false,
+    );
+    let fut_b = process_me_writer_response(
+        MeResponse::Data {
+            flags: 0,
+            data: Bytes::from_static(&[0x22]),
+        },
+        &mut writer_b,
+        ProtoTag::Intermediate,
+        &rng_b,
+        &mut frame_buf_b,
+        &stats,
+        user,
+        Some(1),
+        &bytes_me2c,
+        92,
+        false,
+        false,
+    );
+
+    let (result_a, result_b) = tokio::join!(fut_a, fut_b);
+
+    assert!(
+        matches!(result_a, Err(ProxyError::DataQuotaExceeded { ref user }) if user == "quota-race-user")
+            || matches!(result_a, Ok(_)),
+        "concurrent quota test must complete without panicking"
+    );
+    assert!(
+        matches!(result_b, Err(ProxyError::DataQuotaExceeded { ref user }) if user == "quota-race-user")
+            || matches!(result_b, Ok(_)),
+        "concurrent quota test must complete without panicking"
+    );
+    assert!(
+        stats.get_user_total_octets(user) <= 1,
+        "same-user concurrent middle-relay responses must not overshoot the configured quota"
+    );
+}
+
+#[tokio::test]
+async fn process_me_writer_response_data_does_not_forward_partial_payload_when_remaining_quota_is_smaller_than_message() {
+    let (writer_side, mut reader_side) = duplex(1024);
+    let mut writer = make_crypto_writer(writer_side);
+    let rng = SecureRandom::new();
+    let mut frame_buf = Vec::new();
+    let stats = Stats::new();
+    let bytes_me2c = AtomicU64::new(0);
+
+    stats.add_user_octets_to("partial-quota-user", 3);
+
+    let result = process_me_writer_response(
+        MeResponse::Data {
+            flags: 0,
+            data: Bytes::from(vec![1u8, 2, 3, 4]),
+        },
+        &mut writer,
+        ProtoTag::Intermediate,
+        &rng,
+        &mut frame_buf,
+        &stats,
+        "partial-quota-user",
+        Some(4),
+        &bytes_me2c,
+        90,
+        false,
+        false,
+    )
+    .await;
+
+    assert!(
+        matches!(result, Err(ProxyError::DataQuotaExceeded { user }) if user == "partial-quota-user"),
+        "ME->client runtime path must reject oversized payloads before writing"
+    );
+
+    let mut raw = [0u8; 1];
+    assert!(
+        timeout(TokioDuration::from_millis(100), reader_side.read(&mut raw))
+            .await
+            .is_err(),
+        "oversized payloads must not leak any partial ciphertext to the client stream"
+    );
+}
+
 #[tokio::test]
 async fn middle_relay_abort_midflight_releases_route_gauge() {
     let stats = Arc::new(Stats::new());

src/proxy/relay.rs

@@ -53,16 +53,17 @@
 
 use std::io;
 use std::pin::Pin;
-use std::sync::Arc;
+use std::sync::{Arc, Mutex, OnceLock};
-use std::sync::atomic::{AtomicU64, Ordering};
+use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
 use std::task::{Context, Poll};
 use std::time::Duration;
+use dashmap::DashMap;
 use tokio::io::{
     AsyncRead, AsyncWrite, AsyncWriteExt, ReadBuf, copy_bidirectional_with_sizes,
 };
 use tokio::time::Instant;
 use tracing::{debug, trace, warn};
-use crate::error::Result;
+use crate::error::{ProxyError, Result};
 use crate::stats::Stats;
 use crate::stream::BufferPool;
 
@@ -205,6 +206,8 @@ struct StatsIo<S> {
     counters: Arc<SharedCounters>,
     stats: Arc<Stats>,
     user: String,
+    quota_limit: Option<u64>,
+    quota_exceeded: Arc<AtomicBool>,
     epoch: Instant,
 }
 
@@ -214,11 +217,62 @@ impl<S> StatsIo<S> {
         counters: Arc<SharedCounters>,
         stats: Arc<Stats>,
         user: String,
+        quota_limit: Option<u64>,
+        quota_exceeded: Arc<AtomicBool>,
         epoch: Instant,
     ) -> Self {
         // Mark initial activity so the watchdog doesn't fire before data flows
         counters.touch(Instant::now(), epoch);
-        Self { inner, counters, stats, user, epoch }
+        Self {
+            inner,
+            counters,
+            stats,
+            user,
+            quota_limit,
+            quota_exceeded,
+            epoch,
+        }
+    }
+}
+
+#[derive(Debug)]
+struct QuotaIoSentinel;
+
+impl std::fmt::Display for QuotaIoSentinel {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str("user data quota exceeded")
+    }
+}
+
+impl std::error::Error for QuotaIoSentinel {}
+
+fn quota_io_error() -> io::Error {
+    io::Error::new(io::ErrorKind::PermissionDenied, QuotaIoSentinel)
+}
+
+fn is_quota_io_error(err: &io::Error) -> bool {
+    err.kind() == io::ErrorKind::PermissionDenied
+        && err
+            .get_ref()
+            .and_then(|source| source.downcast_ref::<QuotaIoSentinel>())
+            .is_some()
+}
+
+static QUOTA_USER_LOCKS: OnceLock<DashMap<String, Arc<Mutex<()>>>> = OnceLock::new();
+
+fn quota_user_lock(user: &str) -> Arc<Mutex<()>> {
+    let locks = QUOTA_USER_LOCKS.get_or_init(DashMap::new);
+    if let Some(existing) = locks.get(user) {
+        return Arc::clone(existing.value());
+    }
+
+    let created = Arc::new(Mutex::new(()));
+    match locks.entry(user.to_string()) {
+        dashmap::mapref::entry::Entry::Occupied(entry) => Arc::clone(entry.get()),
+        dashmap::mapref::entry::Entry::Vacant(entry) => {
+            entry.insert(Arc::clone(&created));
+            created
+        }
     }
 }
 
@@ -229,6 +283,32 @@ impl<S: AsyncRead + Unpin> AsyncRead for StatsIo<S> {
         buf: &mut ReadBuf<'_>,
     ) -> Poll<io::Result<()>> {
         let this = self.get_mut();
+        if this.quota_exceeded.load(Ordering::Relaxed) {
+            return Poll::Ready(Err(quota_io_error()));
+        }
+
+        let quota_lock = this
+            .quota_limit
+            .is_some()
+            .then(|| quota_user_lock(&this.user));
+        let _quota_guard = if let Some(lock) = quota_lock.as_ref() {
+            match lock.try_lock() {
+                Ok(guard) => Some(guard),
+                Err(_) => {
+                    cx.waker().wake_by_ref();
+                    return Poll::Pending;
+                }
+            }
+        } else {
+            None
+        };
+
+        if let Some(limit) = this.quota_limit
+            && this.stats.get_user_total_octets(&this.user) >= limit
+        {
+            this.quota_exceeded.store(true, Ordering::Relaxed);
+            return Poll::Ready(Err(quota_io_error()));
+        }
         let before = buf.filled().len();
 
         match Pin::new(&mut this.inner).poll_read(cx, buf) {
@@ -243,6 +323,13 @@ impl<S: AsyncRead + Unpin> AsyncRead for StatsIo<S> {
                     this.stats.add_user_octets_from(&this.user, n as u64);
                     this.stats.increment_user_msgs_from(&this.user);
 
+                    if let Some(limit) = this.quota_limit
+                        && this.stats.get_user_total_octets(&this.user) >= limit
+                    {
+                        this.quota_exceeded.store(true, Ordering::Relaxed);
+                        return Poll::Ready(Err(quota_io_error()));
+                    }
+
                     trace!(user = %this.user, bytes = n, "C->S");
                 }
                 Poll::Ready(Ok(()))
@@ -259,8 +346,46 @@ impl<S: AsyncWrite + Unpin> AsyncWrite for StatsIo<S> {
         buf: &[u8],
     ) -> Poll<io::Result<usize>> {
         let this = self.get_mut();
+        if this.quota_exceeded.load(Ordering::Relaxed) {
+            return Poll::Ready(Err(quota_io_error()));
+        }
 
-        match Pin::new(&mut this.inner).poll_write(cx, buf) {
+        let quota_lock = this
+            .quota_limit
+            .is_some()
+            .then(|| quota_user_lock(&this.user));
+        let _quota_guard = if let Some(lock) = quota_lock.as_ref() {
+            match lock.try_lock() {
+                Ok(guard) => Some(guard),
+                Err(_) => {
+                    cx.waker().wake_by_ref();
+                    return Poll::Pending;
+                }
+            }
+        } else {
+            None
+        };
+
+        let write_buf = if let Some(limit) = this.quota_limit {
+            let used = this.stats.get_user_total_octets(&this.user);
+            if used >= limit {
+                this.quota_exceeded.store(true, Ordering::Relaxed);
+                return Poll::Ready(Err(quota_io_error()));
+            }
+
+            let remaining = (limit - used) as usize;
+            if buf.len() > remaining {
+                // Fail closed: do not emit partial S->C payload when remaining
+                // quota cannot accommodate the pending write request.
+                this.quota_exceeded.store(true, Ordering::Relaxed);
+                return Poll::Ready(Err(quota_io_error()));
+            }
+            buf
+        } else {
+            buf
+        };
+
+        match Pin::new(&mut this.inner).poll_write(cx, write_buf) {
             Poll::Ready(Ok(n)) => {
                 if n > 0 {
                     // S→C: data written to client
@@ -271,6 +396,13 @@ impl<S: AsyncWrite + Unpin> AsyncWrite for StatsIo<S> {
                     this.stats.add_user_octets_to(&this.user, n as u64);
                     this.stats.increment_user_msgs_to(&this.user);
 
+                    if let Some(limit) = this.quota_limit
+                        && this.stats.get_user_total_octets(&this.user) >= limit
+                    {
+                        this.quota_exceeded.store(true, Ordering::Relaxed);
+                        return Poll::Ready(Err(quota_io_error()));
+                    }
+
                     trace!(user = %this.user, bytes = n, "S->C");
                 }
                 Poll::Ready(Ok(n))
@@ -307,7 +439,8 @@ impl<S: AsyncWrite + Unpin> AsyncWrite for StatsIo<S> {
 /// - Per-user stats: bytes and ops counted per direction
 /// - Periodic rate logging: every 10 seconds when active
 /// - Clean shutdown: both write sides are shut down on exit
-/// - Error propagation: I/O errors are returned as `ProxyError::Io`
+/// - Error propagation: quota exits return `ProxyError::DataQuotaExceeded`,
+///   other I/O failures are returned as `ProxyError::Io`
 pub async fn relay_bidirectional<CR, CW, SR, SW>(
     client_reader: CR,
     client_writer: CW,
@@ -317,6 +450,7 @@ pub async fn relay_bidirectional<CR, CW, SR, SW>(
     s2c_buf_size: usize,
     user: &str,
     stats: Arc<Stats>,
+    quota_limit: Option<u64>,
     _buffer_pool: Arc<BufferPool>,
 ) -> Result<()>
 where
@@ -327,6 +461,7 @@ where
 {
     let epoch = Instant::now();
     let counters = Arc::new(SharedCounters::new());
+    let quota_exceeded = Arc::new(AtomicBool::new(false));
     let user_owned = user.to_string();
 
     // ── Combine split halves into bidirectional streams ──────────────
@@ -339,12 +474,15 @@ where
         Arc::clone(&counters),
         Arc::clone(&stats),
         user_owned.clone(),
+        quota_limit,
+        Arc::clone(&quota_exceeded),
         epoch,
     );
 
     // ── Watchdog: activity timeout + periodic rate logging ──────────
     let wd_counters = Arc::clone(&counters);
     let wd_user = user_owned.clone();
+    let wd_quota_exceeded = Arc::clone(&quota_exceeded);
 
     let watchdog = async {
         let mut prev_c2s: u64 = 0;
@@ -356,6 +494,11 @@ where
             let now = Instant::now();
             let idle = wd_counters.idle_duration(now, epoch);
 
+            if wd_quota_exceeded.load(Ordering::Relaxed) {
+                warn!(user = %wd_user, "User data quota reached, closing relay");
+                return;
+            }
+
             // ── Activity timeout ────────────────────────────────────
             if idle >= ACTIVITY_TIMEOUT {
                 let c2s = wd_counters.c2s_bytes.load(Ordering::Relaxed);
@@ -439,6 +582,22 @@ where
             );
             Ok(())
         }
+        Some(Err(e)) if is_quota_io_error(&e) => {
+            let c2s = counters.c2s_bytes.load(Ordering::Relaxed);
+            let s2c = counters.s2c_bytes.load(Ordering::Relaxed);
+            warn!(
+                user = %user_owned,
+                c2s_bytes = c2s,
+                s2c_bytes = s2c,
+                c2s_msgs = c2s_ops,
+                s2c_msgs = s2c_ops,
+                duration_secs = duration.as_secs(),
+                "Data quota reached, closing relay"
+            );
+            Err(ProxyError::DataQuotaExceeded {
+                user: user_owned.clone(),
+            })
+        }
         Some(Err(e)) => {
             // I/O error in one of the directions
             let c2s = counters.c2s_bytes.load(Ordering::Relaxed);
@@ -472,3 +631,7 @@ where
         }
     }
 }
+
+#[cfg(test)]
+#[path = "relay_security_tests.rs"]
+mod security_tests;

src/proxy/relay_security_tests.rs

@@ -0,0 +1,972 @@
+use super::relay_bidirectional;
+use crate::error::ProxyError;
+use crate::stats::Stats;
+use crate::stream::BufferPool;
+use std::future::poll_fn;
+use std::io;
+use std::pin::Pin;
+use std::sync::Arc;
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::Mutex;
+use std::task::{Context, Poll};
+use std::task::Waker;
+use tokio::io::{AsyncRead, ReadBuf};
+use tokio::io::{AsyncReadExt, AsyncWrite, AsyncWriteExt, duplex};
+use tokio::time::{Duration, timeout};
+
+#[tokio::test]
+async fn relay_bidirectional_enforces_live_user_quota() {
+    let stats = Arc::new(Stats::new());
+    let user = "quota-user";
+    stats.add_user_octets_from(user, 6);
+
+    let (mut client_peer, relay_client) = duplex(4096);
+    let (relay_server, mut server_peer) = duplex(4096);
+
+    let (client_reader, client_writer) = tokio::io::split(relay_client);
+    let (server_reader, server_writer) = tokio::io::split(relay_server);
+
+    let relay_task = tokio::spawn(relay_bidirectional(
+        client_reader,
+        client_writer,
+        server_reader,
+        server_writer,
+        1024,
+        1024,
+        user,
+        Arc::clone(&stats),
+        Some(8),
+        Arc::new(BufferPool::new()),
+    ));
+
+    client_peer
+        .write_all(&[0x10, 0x20, 0x30, 0x40])
+        .await
+        .expect("client write must succeed");
+
+    let mut forwarded = [0u8; 4];
+    let _ = timeout(
+        Duration::from_millis(200),
+        server_peer.read_exact(&mut forwarded),
+    )
+    .await;
+
+    let relay_result = timeout(Duration::from_secs(2), relay_task)
+        .await
+        .expect("relay task must finish under quota cutoff")
+        .expect("relay task must not panic");
+
+    assert!(
+        matches!(relay_result, Err(ProxyError::DataQuotaExceeded { ref user }) if user == "quota-user"),
+        "relay must surface a typed quota error once live quota is exceeded"
+    );
+}
+
+#[tokio::test]
+async fn relay_bidirectional_does_not_forward_server_bytes_after_quota_is_exhausted() {
+    let stats = Arc::new(Stats::new());
+    let quota_user = "quota-exhausted-user";
+    stats.add_user_octets_from(quota_user, 1);
+
+    let (mut client_peer, relay_client) = duplex(4096);
+    let (relay_server, mut server_peer) = duplex(4096);
+
+    let (client_reader, client_writer) = tokio::io::split(relay_client);
+    let (server_reader, server_writer) = tokio::io::split(relay_server);
+
+    let relay_task = tokio::spawn(relay_bidirectional(
+        client_reader,
+        client_writer,
+        server_reader,
+        server_writer,
+        1024,
+        1024,
+        quota_user,
+        Arc::clone(&stats),
+        Some(1),
+        Arc::new(BufferPool::new()),
+    ));
+
+    server_peer
+        .write_all(&[0xde, 0xad, 0xbe, 0xef])
+        .await
+        .expect("server write must succeed");
+
+    let mut observed = [0u8; 4];
+    let forwarded = timeout(
+        Duration::from_millis(200),
+        client_peer.read_exact(&mut observed),
+    )
+    .await;
+
+    let relay_result = timeout(Duration::from_secs(2), relay_task)
+        .await
+        .expect("relay task must finish under quota cutoff")
+        .expect("relay task must not panic");
+
+    assert!(
+        !matches!(forwarded, Ok(Ok(n)) if n == observed.len()),
+        "no full server payload should be forwarded once quota is already exhausted"
+    );
+    assert!(
+        matches!(relay_result, Err(ProxyError::DataQuotaExceeded { ref user }) if user == quota_user),
+        "relay must still terminate with a typed quota error"
+    );
+}
+
+#[tokio::test]
+async fn relay_bidirectional_does_not_leak_partial_server_payload_when_remaining_quota_is_smaller_than_write() {
+    let stats = Arc::new(Stats::new());
+    let quota_user = "partial-leak-user";
+    stats.add_user_octets_from(quota_user, 3);
+
+    let (mut client_peer, relay_client) = duplex(4096);
+    let (relay_server, mut server_peer) = duplex(4096);
+
+    let (client_reader, client_writer) = tokio::io::split(relay_client);
+    let (server_reader, server_writer) = tokio::io::split(relay_server);
+
+    let relay_task = tokio::spawn(relay_bidirectional(
+        client_reader,
+        client_writer,
+        server_reader,
+        server_writer,
+        1024,
+        1024,
+        quota_user,
+        Arc::clone(&stats),
+        Some(4),
+        Arc::new(BufferPool::new()),
+    ));
+
+    server_peer
+        .write_all(&[0x11, 0x22, 0x33, 0x44])
+        .await
+        .expect("server write must succeed");
+
+    let mut observed = [0u8; 8];
+    let forwarded = timeout(Duration::from_millis(200), client_peer.read(&mut observed)).await;
+
+    let relay_result = timeout(Duration::from_secs(2), relay_task)
+        .await
+        .expect("relay task must finish under quota cutoff")
+        .expect("relay task must not panic");
+
+    assert!(
+        !matches!(forwarded, Ok(Ok(n)) if n > 0),
+        "quota exhaustion must not leak any partial server payload when remaining quota is smaller than the write"
+    );
+    assert!(
+        matches!(relay_result, Err(ProxyError::DataQuotaExceeded { ref user }) if user == quota_user),
+        "relay must still terminate with a typed quota error"
+    );
+}
+
+#[tokio::test]
+async fn relay_bidirectional_zero_quota_remains_fail_closed_for_server_payloads_under_stress() {
+    let stats = Arc::new(Stats::new());
+    let quota_user = "zero-quota-user";
+
+    for payload_len in [1usize, 16, 512, 4096] {
+        let (mut client_peer, relay_client) = duplex(4096);
+        let (relay_server, mut server_peer) = duplex(4096);
+
+        let (client_reader, client_writer) = tokio::io::split(relay_client);
+        let (server_reader, server_writer) = tokio::io::split(relay_server);
+
+        let relay_task = tokio::spawn(relay_bidirectional(
+            client_reader,
+            client_writer,
+            server_reader,
+            server_writer,
+            1024,
+            1024,
+            quota_user,
+            Arc::clone(&stats),
+            Some(0),
+            Arc::new(BufferPool::new()),
+        ));
+
+        let payload = vec![0x7f; payload_len];
+        let _ = server_peer.write_all(&payload).await;
+
+        let mut observed = vec![0u8; payload_len];
+        let forwarded = timeout(Duration::from_millis(200), client_peer.read(&mut observed)).await;
+
+        let relay_result = timeout(Duration::from_secs(2), relay_task)
+            .await
+            .expect("relay task must finish under zero-quota cutoff")
+            .expect("relay task must not panic");
+
+        assert!(
+            !matches!(forwarded, Ok(Ok(n)) if n > 0),
+            "zero quota must not forward any server bytes for payload_len={payload_len}"
+        );
+        assert!(
+            matches!(relay_result, Err(ProxyError::DataQuotaExceeded { ref user }) if user == quota_user),
+            "zero quota must terminate with the typed quota error for payload_len={payload_len}"
+        );
+    }
+}
+
+#[tokio::test]
+async fn relay_bidirectional_allows_exact_server_payload_at_quota_boundary() {
+    let stats = Arc::new(Stats::new());
+    let quota_user = "exact-boundary-user";
+
+    let (mut client_peer, relay_client) = duplex(4096);
+    let (relay_server, mut server_peer) = duplex(4096);
+
+    let (client_reader, client_writer) = tokio::io::split(relay_client);
+    let (server_reader, server_writer) = tokio::io::split(relay_server);
+
+    let relay_task = tokio::spawn(relay_bidirectional(
+        client_reader,
+        client_writer,
+        server_reader,
+        server_writer,
+        1024,
+        1024,
+        quota_user,
+        Arc::clone(&stats),
+        Some(4),
+        Arc::new(BufferPool::new()),
+    ));
+
+    server_peer
+        .write_all(&[0x91, 0x92, 0x93, 0x94])
+        .await
+        .expect("server write must succeed at exact quota boundary");
+
+    let mut observed = [0u8; 4];
+    client_peer
+        .read_exact(&mut observed)
+        .await
+        .expect("client must receive the full payload at the exact quota boundary");
+    assert_eq!(observed, [0x91, 0x92, 0x93, 0x94]);
+
+    let relay_result = timeout(Duration::from_secs(2), relay_task)
+        .await
+        .expect("relay task must finish after exact boundary delivery")
+        .expect("relay task must not panic");
+
+    assert!(
+        matches!(relay_result, Err(ProxyError::DataQuotaExceeded { ref user }) if user == quota_user),
+        "relay must close with a typed quota error after reaching the exact boundary"
+    );
+}
+
+#[tokio::test]
+async fn relay_bidirectional_does_not_forward_client_bytes_after_quota_is_exhausted() {
+    let stats = Arc::new(Stats::new());
+    let quota_user = "client-exhausted-user";
+    stats.add_user_octets_from(quota_user, 1);
+
+    let (mut client_peer, relay_client) = duplex(4096);
+    let (relay_server, mut server_peer) = duplex(4096);
+
+    let (client_reader, client_writer) = tokio::io::split(relay_client);
+    let (server_reader, server_writer) = tokio::io::split(relay_server);
+
+    let relay_task = tokio::spawn(relay_bidirectional(
+        client_reader,
+        client_writer,
+        server_reader,
+        server_writer,
+        1024,
+        1024,
+        quota_user,
+        Arc::clone(&stats),
+        Some(1),
+        Arc::new(BufferPool::new()),
+    ));
+
+    client_peer
+        .write_all(&[0x51, 0x52, 0x53, 0x54])
+        .await
+        .expect("client write must succeed even when quota is already exhausted");
+
+    let mut observed = [0u8; 4];
+    let forwarded = timeout(
+        Duration::from_millis(200),
+        server_peer.read_exact(&mut observed),
+    )
+    .await;
+
+    let relay_result = timeout(Duration::from_secs(2), relay_task)
+        .await
+        .expect("relay task must finish under quota cutoff")
+        .expect("relay task must not panic");
+
+    assert!(
+        !matches!(forwarded, Ok(Ok(n)) if n == observed.len()),
+        "client payload must not be fully forwarded once quota is already exhausted"
+    );
+    assert!(
+        matches!(relay_result, Err(ProxyError::DataQuotaExceeded { ref user }) if user == quota_user),
+        "relay must still terminate with a typed quota error"
+    );
+}
+
+#[tokio::test]
+async fn relay_bidirectional_server_bytes_remain_blocked_even_under_multiple_payload_sizes() {
+    let stats = Arc::new(Stats::new());
+    let quota_user = "quota-fuzz-user";
+    stats.add_user_octets_from(quota_user, 2);
+
+    for payload_len in [1usize, 32, 1024, 8192] {
+        let (mut client_peer, relay_client) = duplex(4096);
+        let (relay_server, mut server_peer) = duplex(4096);
+
+        let (client_reader, client_writer) = tokio::io::split(relay_client);
+        let (server_reader, server_writer) = tokio::io::split(relay_server);
+
+        let relay_task = tokio::spawn(relay_bidirectional(
+            client_reader,
+            client_writer,
+            server_reader,
+            server_writer,
+            1024,
+            1024,
+            quota_user,
+            Arc::clone(&stats),
+            Some(2),
+            Arc::new(BufferPool::new()),
+        ));
+
+        let payload = vec![0xaa; payload_len];
+        let _ = server_peer.write_all(&payload).await;
+
+        let mut observed = vec![0u8; payload_len];
+        let forwarded = timeout(
+            Duration::from_millis(200),
+            client_peer.read_exact(&mut observed),
+        )
+        .await;
+
+        let relay_result = timeout(Duration::from_secs(2), relay_task)
+            .await
+            .expect("relay task must finish under quota cutoff")
+            .expect("relay task must not panic");
+
+        assert!(
+            !matches!(forwarded, Ok(Ok(n)) if n == payload_len),
+            "quota exhaustion must block full server-to-client forwarding for payload_len={payload_len}"
+        );
+        assert!(
+            matches!(relay_result, Err(ProxyError::DataQuotaExceeded { ref user }) if user == quota_user),
+            "relay must keep returning the typed quota error for payload_len={payload_len}"
+        );
+    }
+}
+
+#[tokio::test]
+async fn relay_bidirectional_terminates_on_activity_timeout() {
+    tokio::time::pause();
+    let stats = Arc::new(Stats::new());
+    let user = "timeout-user";
+
+    let (client_peer, relay_client) = duplex(4096);
+    let (relay_server, server_peer) = duplex(4096);
+
+    let (client_reader, client_writer) = tokio::io::split(relay_client);
+    let (server_reader, server_writer) = tokio::io::split(relay_server);
+
+    let relay_task = tokio::spawn(relay_bidirectional(
+        client_reader,
+        client_writer,
+        server_reader,
+        server_writer,
+        1024,
+        1024,
+        user,
+        Arc::clone(&stats),
+        None, // No quota
+        Arc::new(BufferPool::new()),
+    ));
+
+    // Wait past the activity timeout threshold (1800 seconds) + buffer
+    tokio::time::sleep(Duration::from_secs(1805)).await;
+    
+    // Resume time to process timeouts
+    tokio::time::resume();
+
+    let relay_result = timeout(Duration::from_secs(1), relay_task)
+        .await
+        .expect("relay task must finish inside bounded timeout due to inactivity cutoff")
+        .expect("relay task must not panic");
+
+    assert!(
+        relay_result.is_ok(),
+        "relay should complete successfully on scheduled inactivity timeout"
+    );
+    
+    // Verify client/server sockets are closed
+    drop(client_peer);
+    drop(server_peer);
+}
+
+#[tokio::test]
+async fn relay_bidirectional_watchdog_resists_premature_execution() {
+    tokio::time::pause();
+    let stats = Arc::new(Stats::new());
+    let user = "activity-user";
+
+    let (mut client_peer, relay_client) = duplex(4096);
+    let (relay_server, server_peer) = duplex(4096);
+
+    let (client_reader, client_writer) = tokio::io::split(relay_client);
+    let (server_reader, server_writer) = tokio::io::split(relay_server);
+
+    let mut relay_task = tokio::spawn(relay_bidirectional(
+        client_reader,
+        client_writer,
+        server_reader,
+        server_writer,
+        1024,
+        1024,
+        user,
+        Arc::clone(&stats),
+        None,
+        Arc::new(BufferPool::new()),
+    ));
+
+    // Advance by half the timeout
+    tokio::time::sleep(Duration::from_secs(900)).await;
+
+    // Provide activity
+    client_peer
+        .write_all(&[0xaa, 0xbb])
+        .await
+        .expect("client write must succeed");
+    client_peer.flush().await.unwrap();
+
+    // Advance by another half (total time since start is 1800, but since last activity is 900)
+    tokio::time::sleep(Duration::from_secs(900)).await;
+
+    tokio::time::resume();
+
+    // Re-evaluating the task, it should NOT have timed out and still be pending
+    let relay_result = timeout(Duration::from_millis(100), &mut relay_task).await;
+    assert!(
+        relay_result.is_err(),
+        "Relay must not exit prematurely as long as activity was received before timeout"
+    );
+    
+    // Explicitly drop sockets to cleanly shut down relay loop
+    drop(client_peer);
+    drop(server_peer);
+    
+    let completion = timeout(Duration::from_secs(1), relay_task).await
+        .expect("relay task must complete securely after client disconnection")
+        .expect("relay task must not panic");
+    assert!(completion.is_ok(), "relay exits clean");
+}
+
+#[tokio::test]
+async fn relay_bidirectional_half_closure_terminates_cleanly() {
+    let stats = Arc::new(Stats::new());
+    let (client_peer, relay_client) = duplex(4096);
+    let (relay_server, server_peer) = duplex(4096);
+    let (client_reader, client_writer) = tokio::io::split(relay_client);
+    let (server_reader, server_writer) = tokio::io::split(relay_server);
+
+    let relay_task = tokio::spawn(relay_bidirectional(
+        client_reader, client_writer, server_reader, server_writer, 1024, 1024, "half-close", stats, None, Arc::new(BufferPool::new()),
+    ));
+    
+    // Half closure: drop the client completely but leave the server active.
+    drop(client_peer);
+    
+    // Check that we don't immediately crash. Bidirectional relay stays open for the server -> client flush.
+    // Eventually dropping the server cleanly closes the task.
+    drop(server_peer);
+    timeout(Duration::from_secs(1), relay_task).await.unwrap().unwrap().unwrap();
+}
+
+#[tokio::test]
+async fn relay_bidirectional_zero_length_noise_fuzzing() {
+    let stats = Arc::new(Stats::new());
+    let (mut client_peer, relay_client) = duplex(4096);
+    let (relay_server, mut server_peer) = duplex(4096);
+    let (client_reader, client_writer) = tokio::io::split(relay_client);
+    let (server_reader, server_writer) = tokio::io::split(relay_server);
+
+    let relay_task = tokio::spawn(relay_bidirectional(
+        client_reader, client_writer, server_reader, server_writer, 1024, 1024, "fuzz", stats, None, Arc::new(BufferPool::new()),
+    ));
+
+    // Flood with zero-length payloads (edge cases in stream framing logic sometimes loop)
+    for _ in 0..100 {
+        client_peer.write_all(&[]).await.unwrap();
+    }
+    client_peer.write_all(&[1, 2, 3]).await.unwrap();
+    client_peer.flush().await.unwrap();
+    
+    let mut buf = [0u8; 3];
+    server_peer.read_exact(&mut buf).await.unwrap();
+    assert_eq!(&buf, &[1, 2, 3]);
+    
+    drop(client_peer);
+    drop(server_peer);
+    timeout(Duration::from_secs(1), relay_task).await.unwrap().unwrap().unwrap();
+}
+
+#[tokio::test]
+async fn relay_bidirectional_asymmetric_backpressure() {
+    let stats = Arc::new(Stats::new());
+    // Give the client stream an extremely narrow throughput limit explicitly
+    let (client_peer, relay_client) = duplex(1024); 
+    let (relay_server, mut server_peer) = duplex(4096);
+    let (client_reader, client_writer) = tokio::io::split(relay_client);
+    let (server_reader, server_writer) = tokio::io::split(relay_server);
+
+    let relay_task = tokio::spawn(relay_bidirectional(
+        client_reader, client_writer, server_reader, server_writer, 1024, 1024, "slowloris", stats, None, Arc::new(BufferPool::new()),
+    ));
+
+    let payload = vec![0xba; 65536]; // 64k payload
+    
+    // Server attempts to shove 64KB into a relay whose client pipe only holds 1KB!
+    let write_res = tokio::time::timeout(Duration::from_millis(50), server_peer.write_all(&payload)).await;
+    
+    assert!(
+        write_res.is_err(), 
+        "Relay backpressure MUST halt the server writer from unbounded buffering when client stream is full!"
+    );
+    
+    drop(client_peer);
+    drop(server_peer);
+    
+    let completion = timeout(Duration::from_secs(1), relay_task).await.unwrap().unwrap();
+    assert!(
+        completion.is_ok() || completion.is_err(), 
+        "Task must unwind reliably (either Ok or BrokenPipe Err) when dropped despite active backpressure locks"
+    );
+}
+
+use rand::{Rng, SeedableRng, rngs::StdRng};
+
+#[tokio::test]
+async fn relay_bidirectional_light_fuzzing_temporal_jitter() {
+    tokio::time::pause();
+    let stats = Arc::new(Stats::new());
+    let (mut client_peer, relay_client) = duplex(4096);
+    let (relay_server, server_peer) = duplex(4096);
+    let (client_reader, client_writer) = tokio::io::split(relay_client);
+    let (server_reader, server_writer) = tokio::io::split(relay_server);
+
+    let mut relay_task = tokio::spawn(relay_bidirectional(
+        client_reader, client_writer, server_reader, server_writer, 1024, 1024, "fuzz-user", stats, None, Arc::new(BufferPool::new()),
+    ));
+
+    let mut rng = StdRng::seed_from_u64(0xDEADBEEF);
+    
+    for _ in 0..10 {
+        // Vary timing significantly up to 1600 seconds (limit is 1800s)
+        let jitter = rng.random_range(100..1600); 
+        tokio::time::sleep(Duration::from_secs(jitter)).await;
+        
+        client_peer.write_all(&[0x11]).await.unwrap();
+        client_peer.flush().await.unwrap();
+        
+        // Ensure task has not died
+        let res = timeout(Duration::from_millis(10), &mut relay_task).await;
+        assert!(res.is_err(), "Relay must remain open indefinitely under light temporal fuzzing with active jitter pulses");
+    }
+    
+    drop(client_peer);
+    drop(server_peer);
+    timeout(Duration::from_secs(1), relay_task).await.unwrap().unwrap().unwrap();
+}
+
+struct FaultyReader {
+    error_once: Option<io::Error>,
+}
+
+struct TwoPartyGate {
+    arrivals: AtomicUsize,
+    total_bytes: AtomicUsize,
+    wakers: Mutex<Vec<Waker>>,
+}
+
+impl TwoPartyGate {
+    fn new() -> Self {
+        Self {
+            arrivals: AtomicUsize::new(0),
+            total_bytes: AtomicUsize::new(0),
+            wakers: Mutex::new(Vec::new()),
+        }
+    }
+
+    fn arrive_or_park(&self, cx: &mut Context<'_>) -> bool {
+        if self.arrivals.load(Ordering::Relaxed) >= 2 {
+            return true;
+        }
+
+        let prev = self.arrivals.fetch_add(1, Ordering::AcqRel);
+        if prev + 1 >= 2 {
+            let mut wakers = self.wakers.lock().unwrap_or_else(|p| p.into_inner());
+            for waker in wakers.drain(..) {
+                waker.wake();
+            }
+            true
+        } else {
+            let mut wakers = self.wakers.lock().unwrap_or_else(|p| p.into_inner());
+            wakers.push(cx.waker().clone());
+            false
+        }
+    }
+
+    fn total_bytes(&self) -> usize {
+        self.total_bytes.load(Ordering::Relaxed)
+    }
+}
+
+struct GateWriter {
+    gate: Arc<TwoPartyGate>,
+    entered: bool,
+}
+
+impl GateWriter {
+    fn new(gate: Arc<TwoPartyGate>) -> Self {
+        Self {
+            gate,
+            entered: false,
+        }
+    }
+}
+
+impl AsyncWrite for GateWriter {
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        if !self.entered {
+            self.entered = true;
+        }
+
+        if !self.gate.arrive_or_park(cx) {
+            return Poll::Pending;
+        }
+
+        self.gate
+            .total_bytes
+            .fetch_add(buf.len(), Ordering::Relaxed);
+        Poll::Ready(Ok(buf.len()))
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        Poll::Ready(Ok(()))
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        Poll::Ready(Ok(()))
+    }
+}
+
+struct GateReader {
+    gate: Arc<TwoPartyGate>,
+    entered: bool,
+    emitted: bool,
+}
+
+impl GateReader {
+    fn new(gate: Arc<TwoPartyGate>) -> Self {
+        Self {
+            gate,
+            entered: false,
+            emitted: false,
+        }
+    }
+}
+
+impl AsyncRead for GateReader {
+    fn poll_read(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut ReadBuf<'_>,
+    ) -> Poll<io::Result<()>> {
+        if self.emitted {
+            return Poll::Ready(Ok(()));
+        }
+
+        if !self.entered {
+            self.entered = true;
+        }
+
+        if !self.gate.arrive_or_park(cx) {
+            return Poll::Pending;
+        }
+
+        buf.put_slice(&[0x42]);
+        self.gate.total_bytes.fetch_add(1, Ordering::Relaxed);
+        self.emitted = true;
+        Poll::Ready(Ok(()))
+    }
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn adversarial_concurrent_quota_write_race_does_not_overshoot_limit() {
+    let stats = Arc::new(Stats::new());
+    let gate = Arc::new(TwoPartyGate::new());
+    let user = "concurrent-quota-write".to_string();
+
+    let writer_a = super::StatsIo::new(
+        GateWriter::new(Arc::clone(&gate)),
+        Arc::new(super::SharedCounters::new()),
+        Arc::clone(&stats),
+        user.clone(),
+        Some(1),
+        Arc::new(std::sync::atomic::AtomicBool::new(false)),
+        tokio::time::Instant::now(),
+    );
+
+    let writer_b = super::StatsIo::new(
+        GateWriter::new(Arc::clone(&gate)),
+        Arc::new(super::SharedCounters::new()),
+        Arc::clone(&stats),
+        user.clone(),
+        Some(1),
+        Arc::new(std::sync::atomic::AtomicBool::new(false)),
+        tokio::time::Instant::now(),
+    );
+
+    let task_a = tokio::spawn(async move {
+        let mut w = writer_a;
+        AsyncWriteExt::write_all(&mut w, &[0x01]).await
+    });
+    let task_b = tokio::spawn(async move {
+        let mut w = writer_b;
+        AsyncWriteExt::write_all(&mut w, &[0x02]).await
+    });
+
+    let (res_a, res_b) = tokio::join!(task_a, task_b);
+    let _ = res_a.expect("task a must join");
+    let _ = res_b.expect("task b must join");
+
+    assert!(
+        gate.total_bytes() <= 1,
+        "concurrent same-user writes must not forward more than one byte under quota=1"
+    );
+    assert!(
+        stats.get_user_total_octets(&user) <= 1,
+        "concurrent same-user writes must not account over limit"
+    );
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn adversarial_concurrent_quota_read_race_does_not_overshoot_limit() {
+    let stats = Arc::new(Stats::new());
+    let gate = Arc::new(TwoPartyGate::new());
+    let user = "concurrent-quota-read".to_string();
+
+    let reader_a = super::StatsIo::new(
+        GateReader::new(Arc::clone(&gate)),
+        Arc::new(super::SharedCounters::new()),
+        Arc::clone(&stats),
+        user.clone(),
+        Some(1),
+        Arc::new(std::sync::atomic::AtomicBool::new(false)),
+        tokio::time::Instant::now(),
+    );
+
+    let reader_b = super::StatsIo::new(
+        GateReader::new(Arc::clone(&gate)),
+        Arc::new(super::SharedCounters::new()),
+        Arc::clone(&stats),
+        user.clone(),
+        Some(1),
+        Arc::new(std::sync::atomic::AtomicBool::new(false)),
+        tokio::time::Instant::now(),
+    );
+
+    let task_a = tokio::spawn(async move {
+        let mut r = reader_a;
+        let mut one = [0u8; 1];
+        AsyncReadExt::read_exact(&mut r, &mut one).await
+    });
+    let task_b = tokio::spawn(async move {
+        let mut r = reader_b;
+        let mut one = [0u8; 1];
+        AsyncReadExt::read_exact(&mut r, &mut one).await
+    });
+
+    let (res_a, res_b) = tokio::join!(task_a, task_b);
+    let _ = res_a.expect("task a must join");
+    let _ = res_b.expect("task b must join");
+
+    assert!(
+        gate.total_bytes() <= 1,
+        "concurrent same-user reads must not consume more than one byte under quota=1"
+    );
+    assert!(
+        stats.get_user_total_octets(&user) <= 1,
+        "concurrent same-user reads must not account over limit"
+    );
+}
+
+#[tokio::test]
+async fn stress_same_user_quota_parallel_relays_never_exceed_limit() {
+    let stats = Arc::new(Stats::new());
+    let user = "parallel-quota-user";
+
+    for _ in 0..128 {
+        let (mut client_peer_a, relay_client_a) = duplex(256);
+        let (relay_server_a, mut server_peer_a) = duplex(256);
+        let (mut client_peer_b, relay_client_b) = duplex(256);
+        let (relay_server_b, mut server_peer_b) = duplex(256);
+
+        let (client_reader_a, client_writer_a) = tokio::io::split(relay_client_a);
+        let (server_reader_a, server_writer_a) = tokio::io::split(relay_server_a);
+        let (client_reader_b, client_writer_b) = tokio::io::split(relay_client_b);
+        let (server_reader_b, server_writer_b) = tokio::io::split(relay_server_b);
+
+        let relay_a = tokio::spawn(relay_bidirectional(
+            client_reader_a,
+            client_writer_a,
+            server_reader_a,
+            server_writer_a,
+            64,
+            64,
+            user,
+            Arc::clone(&stats),
+            Some(1),
+            Arc::new(BufferPool::new()),
+        ));
+
+        let relay_b = tokio::spawn(relay_bidirectional(
+            client_reader_b,
+            client_writer_b,
+            server_reader_b,
+            server_writer_b,
+            64,
+            64,
+            user,
+            Arc::clone(&stats),
+            Some(1),
+            Arc::new(BufferPool::new()),
+        ));
+
+        let _ = tokio::join!(
+            client_peer_a.write_all(&[0x01]),
+            server_peer_a.write_all(&[0x02]),
+            client_peer_b.write_all(&[0x03]),
+            server_peer_b.write_all(&[0x04]),
+        );
+
+        let _ = timeout(Duration::from_millis(50), poll_fn(|cx| {
+            let mut one = [0u8; 1];
+            let _ = Pin::new(&mut client_peer_a).poll_read(cx, &mut ReadBuf::new(&mut one));
+            Poll::Ready(())
+        }))
+        .await;
+
+        drop(client_peer_a);
+        drop(server_peer_a);
+        drop(client_peer_b);
+        drop(server_peer_b);
+
+        let _ = timeout(Duration::from_secs(1), relay_a).await;
+        let _ = timeout(Duration::from_secs(1), relay_b).await;
+
+        assert!(
+            stats.get_user_total_octets(user) <= 1,
+            "parallel relays must not exceed configured quota"
+        );
+    }
+}
+
+impl FaultyReader {
+    fn permission_denied_with_message(message: impl Into<String>) -> Self {
+        Self {
+            error_once: Some(io::Error::new(io::ErrorKind::PermissionDenied, message.into())),
+        }
+    }
+}
+
+impl AsyncRead for FaultyReader {
+    fn poll_read(
+        mut self: Pin<&mut Self>,
+        _cx: &mut Context<'_>,
+        _buf: &mut ReadBuf<'_>,
+    ) -> Poll<io::Result<()>> {
+        if let Some(err) = self.error_once.take() {
+            return Poll::Ready(Err(err));
+        }
+        Poll::Ready(Ok(()))
+    }
+}
+
+#[tokio::test]
+async fn relay_bidirectional_does_not_misclassify_transport_permission_denied_as_quota() {
+    let stats = Arc::new(Stats::new());
+    let (client_peer, relay_client) = duplex(4096);
+    let (client_reader, client_writer) = tokio::io::split(relay_client);
+
+    let relay_result = relay_bidirectional(
+        client_reader,
+        client_writer,
+        FaultyReader::permission_denied_with_message("user data quota exceeded"),
+        tokio::io::sink(),
+        1024,
+        1024,
+        "non-quota-permission-denied",
+        Arc::clone(&stats),
+        None,
+        Arc::new(BufferPool::new()),
+    )
+    .await;
+
+    drop(client_peer);
+
+    assert!(
+        matches!(relay_result, Err(ProxyError::Io(ref err)) if err.kind() == io::ErrorKind::PermissionDenied),
+        "non-quota transport PermissionDenied errors must remain IO errors"
+    );
+}
+
+#[tokio::test]
+async fn relay_bidirectional_light_fuzz_permission_denied_messages_remain_io_errors() {
+    let mut rng = StdRng::seed_from_u64(0xA11CE0B5);
+
+    for i in 0..128u64 {
+        let stats = Arc::new(Stats::new());
+        let (client_peer, relay_client) = duplex(1024);
+        let (client_reader, client_writer) = tokio::io::split(relay_client);
+
+        let random_len = rng.random_range(1..=48);
+        let mut msg = String::with_capacity(random_len);
+        for _ in 0..random_len {
+            let ch = (b'a' + (rng.random::<u8>() % 26)) as char;
+            msg.push(ch);
+        }
+        // Include the legacy quota string in a subset of fuzz cases to validate
+        // collision resistance against message-based classification.
+        if i % 7 == 0 {
+            msg = "user data quota exceeded".to_string();
+        }
+
+        let relay_result = relay_bidirectional(
+            client_reader,
+            client_writer,
+            FaultyReader::permission_denied_with_message(msg),
+            tokio::io::sink(),
+            1024,
+            1024,
+            "fuzz-perm-denied",
+            Arc::clone(&stats),
+            None,
+            Arc::new(BufferPool::new()),
+        )
+        .await;
+
+        drop(client_peer);
+
+        assert!(
+            matches!(relay_result, Err(ProxyError::Io(ref err)) if err.kind() == io::ErrorKind::PermissionDenied),
+            "transport PermissionDenied case must stay typed as IO regardless of message content"
+        );
+    }
+}

src/tls_front/emulator.rs

@@ -103,7 +103,7 @@ pub fn build_emulated_server_hello(
     cached: &CachedTlsData,
     use_full_cert_payload: bool,
     rng: &SecureRandom,
-    _alpn: Option<Vec<u8>>,
+    alpn: Option<Vec<u8>>,
     new_session_tickets: u8,
 ) -> Vec<u8> {
     // --- ServerHello ---
@@ -198,8 +198,22 @@ pub fn build_emulated_server_hello(
     }
 
     let mut app_data = Vec::new();
+    let alpn_marker = alpn
+        .as_ref()
+        .filter(|p| !p.is_empty() && p.len() <= u8::MAX as usize)
+        .map(|proto| {
+            let proto_list_len = 1usize + proto.len();
+            let ext_data_len = 2usize + proto_list_len;
+            let mut marker = Vec::with_capacity(4 + ext_data_len);
+            marker.extend_from_slice(&0x0010u16.to_be_bytes());
+            marker.extend_from_slice(&(ext_data_len as u16).to_be_bytes());
+            marker.extend_from_slice(&(proto_list_len as u16).to_be_bytes());
+            marker.push(proto.len() as u8);
+            marker.extend_from_slice(proto);
+            marker
+        });
     let mut payload_offset = 0usize;
-    for size in sizes {
+    for (idx, size) in sizes.into_iter().enumerate() {
         let mut rec = Vec::with_capacity(5 + size);
         rec.push(TLS_RECORD_APPLICATION);
         rec.extend_from_slice(&TLS_VERSION);
@@ -224,7 +238,20 @@ pub fn build_emulated_server_hello(
             }
         } else if size > 17 {
             let body_len = size - 17;
-            rec.extend_from_slice(&rng.bytes(body_len));
+            let mut body = Vec::with_capacity(body_len);
+            if idx == 0 && let Some(marker) = &alpn_marker {
+                if marker.len() <= body_len {
+                    body.extend_from_slice(marker);
+                    if body_len > marker.len() {
+                        body.extend_from_slice(&rng.bytes(body_len - marker.len()));
+                    }
+                } else {
+                    body.extend_from_slice(&rng.bytes(body_len));
+                }
+            } else {
+                body.extend_from_slice(&rng.bytes(body_len));
+            }
+            rec.extend_from_slice(&body);
             rec.push(0x16); // inner content type marker (handshake)
             rec.extend_from_slice(&rng.bytes(16)); // AEAD-like tag
         } else {
@@ -236,8 +263,9 @@ pub fn build_emulated_server_hello(
     // --- Combine ---
     // Optional NewSessionTicket mimic records (opaque ApplicationData for fingerprint).
     let mut tickets = Vec::new();
-    if new_session_tickets > 0 {
+    let ticket_count = new_session_tickets.min(4);
-        for _ in 0..new_session_tickets {
+    if ticket_count > 0 {
+        for _ in 0..ticket_count {
             let ticket_len: usize = rng.range(48) + 48;
             let mut rec = Vec::with_capacity(5 + ticket_len);
             rec.push(TLS_RECORD_APPLICATION);
@@ -264,6 +292,10 @@ pub fn build_emulated_server_hello(
     response
 }
 
+#[cfg(test)]
+#[path = "emulator_security_tests.rs"]
+mod security_tests;
+
 #[cfg(test)]
 mod tests {
     use std::time::SystemTime;

src/tls_front/emulator_security_tests.rs

@@ -0,0 +1,136 @@
+use std::time::SystemTime;
+
+use crate::crypto::SecureRandom;
+use crate::protocol::constants::{TLS_RECORD_APPLICATION, TLS_RECORD_CHANGE_CIPHER, TLS_RECORD_HANDSHAKE};
+use crate::tls_front::emulator::build_emulated_server_hello;
+use crate::tls_front::types::{
+    CachedTlsData, ParsedServerHello, TlsBehaviorProfile, TlsCertPayload, TlsProfileSource,
+};
+
+fn make_cached(cert_payload: Option<crate::tls_front::types::TlsCertPayload>) -> CachedTlsData {
+    CachedTlsData {
+        server_hello_template: ParsedServerHello {
+            version: [0x03, 0x03],
+            random: [0u8; 32],
+            session_id: Vec::new(),
+            cipher_suite: [0x13, 0x01],
+            compression: 0,
+            extensions: Vec::new(),
+        },
+        cert_info: None,
+        cert_payload,
+        app_data_records_sizes: vec![64],
+        total_app_data_len: 64,
+        behavior_profile: TlsBehaviorProfile {
+            change_cipher_spec_count: 1,
+            app_data_record_sizes: vec![64],
+            ticket_record_sizes: Vec::new(),
+            source: TlsProfileSource::Default,
+        },
+        fetched_at: SystemTime::now(),
+        domain: "example.com".to_string(),
+    }
+}
+
+fn first_app_data_payload(response: &[u8]) -> &[u8] {
+    let hello_len = u16::from_be_bytes([response[3], response[4]]) as usize;
+    let ccs_start = 5 + hello_len;
+    let ccs_len = u16::from_be_bytes([response[ccs_start + 3], response[ccs_start + 4]]) as usize;
+    let app_start = ccs_start + 5 + ccs_len;
+    let app_len = u16::from_be_bytes([response[app_start + 3], response[app_start + 4]]) as usize;
+    &response[app_start + 5..app_start + 5 + app_len]
+}
+
+#[test]
+fn emulated_server_hello_ignores_oversized_alpn_when_marker_would_not_fit() {
+    let cached = make_cached(None);
+    let rng = SecureRandom::new();
+    let oversized_alpn = vec![0xAB; u8::MAX as usize + 1];
+
+    let response = build_emulated_server_hello(
+        b"secret",
+        &[0x11; 32],
+        &[0x22; 16],
+        &cached,
+        true,
+        &rng,
+        Some(oversized_alpn),
+        0,
+    );
+
+    assert_eq!(response[0], TLS_RECORD_HANDSHAKE);
+    let hello_len = u16::from_be_bytes([response[3], response[4]]) as usize;
+    let ccs_start = 5 + hello_len;
+    assert_eq!(response[ccs_start], TLS_RECORD_CHANGE_CIPHER);
+    let app_start = ccs_start + 6;
+    assert_eq!(response[app_start], TLS_RECORD_APPLICATION);
+
+    let payload = first_app_data_payload(&response);
+    let mut marker_prefix = Vec::new();
+    marker_prefix.extend_from_slice(&0x0010u16.to_be_bytes());
+    marker_prefix.extend_from_slice(&0x0102u16.to_be_bytes());
+    marker_prefix.extend_from_slice(&0x0100u16.to_be_bytes());
+    marker_prefix.push(0xff);
+    marker_prefix.extend_from_slice(&[0xab; 8]);
+    assert!(
+        !payload.starts_with(&marker_prefix),
+        "oversized ALPN must not be partially embedded into the emulated first application record"
+    );
+}
+
+#[test]
+fn emulated_server_hello_embeds_full_alpn_marker_when_body_can_fit() {
+    let cached = make_cached(None);
+    let rng = SecureRandom::new();
+
+    let response = build_emulated_server_hello(
+        b"secret",
+        &[0x31; 32],
+        &[0x41; 16],
+        &cached,
+        true,
+        &rng,
+        Some(b"h2".to_vec()),
+        0,
+    );
+
+    let payload = first_app_data_payload(&response);
+    let expected = [0x00u8, 0x10, 0x00, 0x05, 0x00, 0x03, 0x02, b'h', b'2'];
+    assert!(
+        payload.starts_with(&expected),
+        "when body has enough capacity, emulated first application record must include full ALPN marker"
+    );
+}
+
+#[test]
+fn emulated_server_hello_prefers_cert_payload_over_alpn_marker() {
+    let cert_msg = vec![0x0b, 0x00, 0x00, 0x05, 0x00, 0xaa, 0xbb, 0xcc, 0xdd];
+    let cached = make_cached(Some(TlsCertPayload {
+        cert_chain_der: vec![vec![0x30, 0x01, 0x00]],
+        certificate_message: cert_msg.clone(),
+    }));
+    let rng = SecureRandom::new();
+
+    let response = build_emulated_server_hello(
+        b"secret",
+        &[0x32; 32],
+        &[0x42; 16],
+        &cached,
+        true,
+        &rng,
+        Some(b"h2".to_vec()),
+        0,
+    );
+
+    let payload = first_app_data_payload(&response);
+    let alpn_marker = [0x00u8, 0x10, 0x00, 0x05, 0x00, 0x03, 0x02, b'h', b'2'];
+
+    assert!(
+        payload.starts_with(&cert_msg),
+        "when certificate payload is available, first record must start with cert payload bytes"
+    );
+    assert!(
+        !payload.starts_with(&alpn_marker),
+        "ALPN marker must not displace selected certificate payload"
+    );
+}

src/transport/middle_proxy/mod.rs

@@ -27,6 +27,8 @@ mod health_regression_tests;
 mod health_integration_tests;
 #[cfg(test)]
 mod health_adversarial_tests;
+#[cfg(test)]
+mod send_adversarial_tests;
 
 use bytes::Bytes;
 

src/transport/middle_proxy/pool.rs

@@ -692,6 +692,7 @@ impl MePool {
         }
     }
 
+    #[allow(dead_code)]
     pub(super) fn draining_active_runtime(&self) -> u64 {
         self.draining_active_runtime.load(Ordering::Relaxed)
     }

src/transport/middle_proxy/registry.rs

@@ -454,6 +454,7 @@ impl ConnRegistry {
         true
     }
 
+    #[allow(dead_code)]
     pub(super) async fn non_empty_writer_ids(&self, writer_ids: &[u64]) -> HashSet<u64> {
         let inner = self.inner.read().await;
         let mut out = HashSet::<u64>::with_capacity(writer_ids.len());

src/transport/middle_proxy/send.rs

@@ -372,17 +372,20 @@ impl MePool {
                 }
                 let effective_our_addr = SocketAddr::new(w.source_ip, our_addr.port());
                 let (payload, meta) = build_routed_payload(effective_our_addr);
-                match w.tx.try_send(WriterCommand::Data(payload.clone())) {
+                match w.tx.clone().try_reserve_owned() {
-                    Ok(()) => {
+                    Ok(permit) => {
-                        self.stats.increment_me_writer_pick_success_try_total(pick_mode);
                         if !self.registry.bind_writer(conn_id, w.id, meta).await {
                             debug!(
                                 conn_id,
                                 writer_id = w.id,
-                                "ME writer disappeared before bind commit, retrying"
+                                "ME writer disappeared before bind commit, pruning stale writer"
                             );
+                            drop(permit);
+                            self.remove_writer_and_close_clients(w.id).await;
                             continue;
                         }
+                        permit.send(WriterCommand::Data(payload.clone()));
+                        self.stats.increment_me_writer_pick_success_try_total(pick_mode);
                         if w.generation < self.current_generation() {
                             self.stats.increment_pool_stale_pick_total();
                             debug!(
@@ -422,18 +425,21 @@ impl MePool {
             self.stats.increment_me_writer_pick_blocking_fallback_total();
             let effective_our_addr = SocketAddr::new(w.source_ip, our_addr.port());
             let (payload, meta) = build_routed_payload(effective_our_addr);
-            match w.tx.send(WriterCommand::Data(payload.clone())).await {
+            match w.tx.clone().reserve_owned().await {
-                Ok(()) => {
+                Ok(permit) => {
-                    self.stats
-                        .increment_me_writer_pick_success_fallback_total(pick_mode);
                     if !self.registry.bind_writer(conn_id, w.id, meta).await {
                         debug!(
                             conn_id,
                             writer_id = w.id,
-                            "ME writer disappeared before fallback bind commit, retrying"
+                            "ME writer disappeared before fallback bind commit, pruning stale writer"
                         );
+                        drop(permit);
+                        self.remove_writer_and_close_clients(w.id).await;
                         continue;
                     }
+                    permit.send(WriterCommand::Data(payload.clone()));
+                    self.stats
+                        .increment_me_writer_pick_success_fallback_total(pick_mode);
                     if w.generation < self.current_generation() {
                         self.stats.increment_pool_stale_pick_total();
                     }

src/transport/middle_proxy/send_adversarial_tests.rs

@@ -0,0 +1,263 @@
+use std::collections::HashMap;
+use std::net::{IpAddr, Ipv4Addr, SocketAddr};
+use std::sync::Arc;
+use std::sync::atomic::{AtomicBool, AtomicU8, AtomicU32, AtomicU64, Ordering};
+use std::time::{Duration, Instant};
+
+use tokio::sync::mpsc;
+use tokio_util::sync::CancellationToken;
+
+use super::codec::WriterCommand;
+use super::pool::{MePool, MeWriter, WriterContour};
+use crate::config::{GeneralConfig, MeRouteNoWriterMode, MeSocksKdfPolicy, MeWriterPickMode};
+use crate::crypto::SecureRandom;
+use crate::network::probe::NetworkDecision;
+use crate::stats::Stats;
+
+async fn make_pool() -> (Arc<MePool>, Arc<SecureRandom>) {
+    let general = GeneralConfig {
+        me_route_no_writer_mode: MeRouteNoWriterMode::AsyncRecoveryFailfast,
+        me_route_no_writer_wait_ms: 50,
+        me_writer_pick_mode: MeWriterPickMode::SortedRr,
+        me_deterministic_writer_sort: true,
+        ..GeneralConfig::default()
+    };
+
+    let rng = Arc::new(SecureRandom::new());
+    let pool = MePool::new(
+        None,
+        vec![1u8; 32],
+        None,
+        false,
+        None,
+        Vec::new(),
+        1,
+        None,
+        12,
+        1200,
+        HashMap::new(),
+        HashMap::new(),
+        None,
+        NetworkDecision::default(),
+        None,
+        rng.clone(),
+        Arc::new(Stats::default()),
+        general.me_keepalive_enabled,
+        general.me_keepalive_interval_secs,
+        general.me_keepalive_jitter_secs,
+        general.me_keepalive_payload_random,
+        general.rpc_proxy_req_every,
+        general.me_warmup_stagger_enabled,
+        general.me_warmup_step_delay_ms,
+        general.me_warmup_step_jitter_ms,
+        general.me_reconnect_max_concurrent_per_dc,
+        general.me_reconnect_backoff_base_ms,
+        general.me_reconnect_backoff_cap_ms,
+        general.me_reconnect_fast_retry_count,
+        general.me_single_endpoint_shadow_writers,
+        general.me_single_endpoint_outage_mode_enabled,
+        general.me_single_endpoint_outage_disable_quarantine,
+        general.me_single_endpoint_outage_backoff_min_ms,
+        general.me_single_endpoint_outage_backoff_max_ms,
+        general.me_single_endpoint_shadow_rotate_every_secs,
+        general.me_floor_mode,
+        general.me_adaptive_floor_idle_secs,
+        general.me_adaptive_floor_min_writers_single_endpoint,
+        general.me_adaptive_floor_min_writers_multi_endpoint,
+        general.me_adaptive_floor_recover_grace_secs,
+        general.me_adaptive_floor_writers_per_core_total,
+        general.me_adaptive_floor_cpu_cores_override,
+        general.me_adaptive_floor_max_extra_writers_single_per_core,
+        general.me_adaptive_floor_max_extra_writers_multi_per_core,
+        general.me_adaptive_floor_max_active_writers_per_core,
+        general.me_adaptive_floor_max_warm_writers_per_core,
+        general.me_adaptive_floor_max_active_writers_global,
+        general.me_adaptive_floor_max_warm_writers_global,
+        general.hardswap,
+        general.me_pool_drain_ttl_secs,
+        general.me_pool_drain_threshold,
+        general.effective_me_pool_force_close_secs(),
+        general.me_pool_min_fresh_ratio,
+        general.me_hardswap_warmup_delay_min_ms,
+        general.me_hardswap_warmup_delay_max_ms,
+        general.me_hardswap_warmup_extra_passes,
+        general.me_hardswap_warmup_pass_backoff_base_ms,
+        general.me_bind_stale_mode,
+        general.me_bind_stale_ttl_secs,
+        general.me_secret_atomic_snapshot,
+        general.me_deterministic_writer_sort,
+        general.me_writer_pick_mode,
+        general.me_writer_pick_sample_size,
+        MeSocksKdfPolicy::default(),
+        general.me_writer_cmd_channel_capacity,
+        general.me_route_channel_capacity,
+        general.me_route_backpressure_base_timeout_ms,
+        general.me_route_backpressure_high_timeout_ms,
+        general.me_route_backpressure_high_watermark_pct,
+        general.me_reader_route_data_wait_ms,
+        general.me_health_interval_ms_unhealthy,
+        general.me_health_interval_ms_healthy,
+        general.me_warn_rate_limit_ms,
+        general.me_route_no_writer_mode,
+        general.me_route_no_writer_wait_ms,
+        general.me_route_inline_recovery_attempts,
+        general.me_route_inline_recovery_wait_ms,
+    );
+
+    (pool, rng)
+}
+
+async fn insert_writer(
+    pool: &Arc<MePool>,
+    writer_id: u64,
+    writer_dc: i32,
+    addr: SocketAddr,
+    register_in_registry: bool,
+) -> mpsc::Receiver<WriterCommand> {
+    let (tx, rx) = mpsc::channel::<WriterCommand>(8);
+    let writer = MeWriter {
+        id: writer_id,
+        addr,
+        source_ip: addr.ip(),
+        writer_dc,
+        generation: pool.current_generation(),
+        contour: Arc::new(AtomicU8::new(WriterContour::Active.as_u8())),
+        created_at: Instant::now(),
+        tx: tx.clone(),
+        cancel: CancellationToken::new(),
+        degraded: Arc::new(AtomicBool::new(false)),
+        rtt_ema_ms_x10: Arc::new(AtomicU32::new(0)),
+        draining: Arc::new(AtomicBool::new(false)),
+        draining_started_at_epoch_secs: Arc::new(AtomicU64::new(0)),
+        drain_deadline_epoch_secs: Arc::new(AtomicU64::new(0)),
+        allow_drain_fallback: Arc::new(AtomicBool::new(false)),
+    };
+
+    pool.writers.write().await.push(writer);
+    {
+        let mut map = pool.proxy_map_v4.write().await;
+        map.entry(writer_dc)
+            .or_insert_with(Vec::new)
+            .push((addr.ip(), addr.port()));
+    }
+    pool.rebuild_endpoint_dc_map().await;
+    if register_in_registry {
+        pool.registry.register_writer(writer_id, tx).await;
+    }
+    rx
+}
+
+async fn recv_data_count(rx: &mut mpsc::Receiver<WriterCommand>, budget: Duration) -> usize {
+    let start = Instant::now();
+    let mut data_count = 0usize;
+    while Instant::now().duration_since(start) < budget {
+        let remaining = budget.saturating_sub(Instant::now().duration_since(start));
+        match tokio::time::timeout(remaining.min(Duration::from_millis(10)), rx.recv()).await {
+            Ok(Some(WriterCommand::Data(_))) => data_count += 1,
+            Ok(Some(WriterCommand::DataAndFlush(_))) => data_count += 1,
+            Ok(Some(WriterCommand::Close)) => {}
+            Ok(None) => break,
+            Err(_) => break,
+        }
+    }
+    data_count
+}
+
+#[tokio::test]
+async fn send_proxy_req_does_not_replay_when_first_bind_commit_fails() {
+    let (pool, _rng) = make_pool().await;
+    pool.rr.store(0, Ordering::Relaxed);
+
+    let (conn_id, _rx) = pool.registry.register().await;
+    let mut stale_rx = insert_writer(
+        &pool,
+        10,
+        2,
+        SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, 10)), 443),
+        false,
+    )
+    .await;
+    let mut live_rx = insert_writer(
+        &pool,
+        11,
+        2,
+        SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, 11)), 443),
+        true,
+    )
+    .await;
+
+    let result = pool
+        .send_proxy_req(
+            conn_id,
+            2,
+            SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 30000),
+            SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 443),
+            b"hello",
+            0,
+            None,
+        )
+        .await;
+
+    assert!(result.is_ok());
+    assert_eq!(recv_data_count(&mut stale_rx, Duration::from_millis(50)).await, 0);
+    assert_eq!(recv_data_count(&mut live_rx, Duration::from_millis(50)).await, 1);
+
+    let bound = pool.registry.get_writer(conn_id).await;
+    assert!(bound.is_some());
+    assert_eq!(bound.expect("writer should be bound").writer_id, 11);
+}
+
+#[tokio::test]
+async fn send_proxy_req_prunes_iterative_stale_bind_failures_without_data_replay() {
+    let (pool, _rng) = make_pool().await;
+    pool.rr.store(0, Ordering::Relaxed);
+
+    let (conn_id, _rx) = pool.registry.register().await;
+
+    let mut stale_rx_1 = insert_writer(
+        &pool,
+        21,
+        2,
+        SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 1, 21)), 443),
+        false,
+    )
+    .await;
+    let mut stale_rx_2 = insert_writer(
+        &pool,
+        22,
+        2,
+        SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 1, 22)), 443),
+        false,
+    )
+    .await;
+    let mut live_rx = insert_writer(
+        &pool,
+        23,
+        2,
+        SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 1, 23)), 443),
+        true,
+    )
+    .await;
+
+    let result = pool
+        .send_proxy_req(
+            conn_id,
+            2,
+            SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 30001),
+            SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 443),
+            b"storm",
+            0,
+            None,
+        )
+        .await;
+
+    assert!(result.is_ok());
+    assert_eq!(recv_data_count(&mut stale_rx_1, Duration::from_millis(50)).await, 0);
+    assert_eq!(recv_data_count(&mut stale_rx_2, Duration::from_millis(50)).await, 0);
+    assert_eq!(recv_data_count(&mut live_rx, Duration::from_millis(50)).await, 1);
+
+    let writers = pool.writers.read().await;
+    let writer_ids = writers.iter().map(|w| w.id).collect::<Vec<_>>();
+    drop(writers);
+    assert_eq!(writer_ids, vec![23]);
+}