fix(auth): reject unsigned x-amz-* headers to close CopyObject confused-deputy

A presigned or signed PUT authorized for a single object could be turned
into a server-side copy of any object the signing key can read by adding
an unsigned x-amz-copy-source header, executed as the signer. SigV4
verification only walked the signed-headers list, never the headers that
actually arrived; the meta-header check matched only X-Amz-Meta- and ran
only on the presigned path, so an unsigned x-amz-* header outside the
list was never seen while the router still dispatched the PUT to
CopyObjectHandler.

Reject any x-amz-* request header not covered by the signed headers, on
both the presigned (doesPresignedSignatureMatch) and Authorization-header
(doesSignatureMatch) paths, matching AWS S3. The check tests membership
in the signed set rather than value equality, so a header whose first
value is empty (e.g. {"", "/src/secret"}) cannot slip through.
X-Amz-Content-Sha256 is exempt (payload hash: read from the query for
presigned requests and bound into the string-to-sign for signed ones, so
it is self-protected) and X-Amz-Signature-Age is exempt (an internal
scratch header written after verification, so repeated verification of
the same request stays idempotent). The synthesized X-Amz-Tagging header
in PutObjectTagging is now injected after signature verification.

Tests that previously added x-amz-copy-source and friends after signing
(relying on the vulnerable behavior) now re-sign, mirroring real S3
clients. Adds checkUnsignedHeaders unit cases and TestPresignedVerifyIdempotent.

Reported by Oren Yomtov. Inherited unchanged from upstream minio/minio.
Tracked as SN-2026-011.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/bucket-policy.go

@@ -255,8 +255,8 @@ func getConditionValuesWithTags(r *http.Request, lc string, cred auth.Credential
 	}
 
 	cloneHeader := r.Header.Clone()
-	signatureAge := cloneHeader.Get("x-amz-signature-age")
-	cloneHeader.Del("x-amz-signature-age")
+	signatureAge := cloneHeader.Get(xhttp.AmzSignatureAge)
+	cloneHeader.Del(xhttp.AmzSignatureAge)
 	// The presigned V4 verifier overwrites this internal scratch header after
 	// validating the signature. Ignore a value supplied on every other request
 	// type, where it would otherwise synthesize s3:signatureAge.

cmd/erasure-multipart-upload-checksum_test.go

@@ -157,6 +157,13 @@ func copyPartWithoutChecksumHTTP(t *testing.T, apiRouter http.Handler, creds aut
 	if sourceRange != "" {
 		req.Header.Set(xhttp.AmzCopySourceRange, sourceRange)
 	}
+	// Re-sign so the copy-source x-amz-* headers are covered by the signature,
+	// as real S3 clients send them; the verifier rejects unsigned x-amz-*.
+	if creds.AccessKey != "" && creds.SecretKey != "" {
+		if err := signRequestV4(req, creds.AccessKey, creds.SecretKey); err != nil {
+			t.Fatalf("failed to re-sign UploadPartCopy request: %v", err)
+		}
+	}
 	rec := httptest.NewRecorder()
 	apiRouter.ServeHTTP(rec, req)
 	if rec.Code != http.StatusOK {

cmd/object-copy-checksum_test.go

@@ -62,6 +62,13 @@ func copyChecksumRequest(t *testing.T, apiRouter http.Handler, credentials auth.
 		t.Fatalf("failed to build CopyObject request: %v", err)
 	}
 	req.Header.Set(xhttp.AmzCopySource, SlashSeparator+pathJoin(bucket, source))
+	// Re-sign so x-amz-copy-source is covered by the signature, as real S3
+	// clients send it; the verifier rejects unsigned x-amz-* headers.
+	if credentials.AccessKey != "" && credentials.SecretKey != "" {
+		if err := signRequestV4(req, credentials.AccessKey, credentials.SecretKey); err != nil {
+			t.Fatalf("failed to re-sign CopyObject request: %v", err)
+		}
+	}
 	rec := httptest.NewRecorder()
 	apiRouter.ServeHTTP(rec, req)
 	return rec

cmd/object-copy-federation_test.go

@@ -197,6 +197,13 @@ func federatedCopyRequest(t *testing.T, apiRouter http.Handler, credentials auth
 		t.Fatalf("failed to build federated CopyObject request: %v", err)
 	}
 	req.Header.Set(xhttp.AmzCopySource, SlashSeparator+pathJoin(srcBucket, srcObject))
+	// Re-sign so x-amz-copy-source is covered by the signature, as real S3
+	// clients send it; the verifier rejects unsigned x-amz-* headers.
+	if credentials.AccessKey != "" && credentials.SecretKey != "" {
+		if err := signRequestV4(req, credentials.AccessKey, credentials.SecretKey); err != nil {
+			t.Fatalf("failed to re-sign federated CopyObject request: %v", err)
+		}
+	}
 	rec := httptest.NewRecorder()
 	apiRouter.ServeHTTP(rec, req)
 	return rec

cmd/object-handlers.go

@@ -3658,9 +3658,6 @@ func (api objectAPIHandlers) PutObjectTaggingHandler(w http.ResponseWriter, r *h
 	}
 	tagsStr := tags.String()
 
-	// Set this such that authorization policies can be applied on the object tags.
-	r.Header.Set(xhttp.AmzObjectTagging, tagsStr)
-
 	logger.GetReqInfo(ctx).BucketName = bucket
 	logger.GetReqInfo(ctx).ObjectName = object
 	if s3Error := authenticateRequest(ctx, r, policy.PutObjectTaggingAction); s3Error != ErrNone {
@@ -3668,6 +3665,12 @@ func (api objectAPIHandlers) PutObjectTaggingHandler(w http.ResponseWriter, r *h
 		return
 	}
 
+	// Set this such that authorization policies can be applied on the object
+	// tags. This is derived from the request body, so it must be injected only
+	// after signature verification: the SigV4 verifier now rejects unsigned
+	// x-amz-* request headers, and this synthesized header is never signed.
+	r.Header.Set(xhttp.AmzObjectTagging, tagsStr)
+
 	opts, err := getOpts(ctx, r, bucket, object)
 	if err != nil {
 		writeErrorResponse(ctx, w, toAPIError(ctx, err), r.URL)

cmd/object-handlers_test.go

@@ -1799,6 +1799,12 @@ func testAPICopyObjectPartHandlerSanity(obj ObjectLayer, instanceType, bucketNam
 		req.Header.Set("X-Amz-Copy-Source", url.QueryEscape(pathJoin(bucketName, objectName)))
 		req.Header.Set("X-Amz-Copy-Source-Range", fmt.Sprintf("bytes=%d-%d", a, b))
 
+		// Re-sign so the copy-source x-amz-* headers are covered by the
+		// signature, as real clients do; the verifier rejects unsigned x-amz-*.
+		if err = signRequestV4(req, credentials.AccessKey, credentials.SecretKey); err != nil {
+			t.Fatalf("Test failed to re-sign HTTP request for copy object part: <ERROR> %v", err)
+		}
+
 		// Since `apiRouter` satisfies `http.Handler` it has a ServeHTTP to execute the logic of the handler.
 		// Call the ServeHTTP to execute the handler, `func (api objectAPIHandlers) CopyObjectHandler` handles the request.
 		a = globalMinPartSize + 1
@@ -2200,6 +2206,15 @@ func testAPICopyObjectPartHandler(obj ObjectLayer, instanceType, bucketName stri
 			}
 		}
 
+		// Re-sign so the copy-source x-amz-* headers set above are covered by
+		// the signature, as real clients do; the verifier rejects unsigned
+		// x-amz-* headers.
+		if testCase.accessKey != "" && testCase.secretKey != "" {
+			if err = signRequestV4(req, testCase.accessKey, testCase.secretKey); err != nil {
+				t.Fatalf("Test %d: Failed to re-sign HTTP request for copy Object: <ERROR> %v", i+1, err)
+			}
+		}
+
 		// Since `apiRouter` satisfies `http.Handler` it has a ServeHTTP to execute the logic of the handler.
 		// Call the ServeHTTP to execute the handler, `func (api objectAPIHandlers) CopyObjectHandler` handles the request.
 		apiRouter.ServeHTTP(rec, req)
@@ -2626,6 +2641,16 @@ func testAPICopyObjectHandler(obj ObjectLayer, instanceType, bucketName string,
 		if testCase.metadataGarbage {
 			req.Header.Set("X-Amz-Metadata-Directive", "Unknown")
 		}
+		// The x-amz-copy-source and related x-amz-* headers set above must be
+		// part of the SigV4 signature, exactly as real S3 clients send them.
+		// Re-sign now that they are present; the verifier rejects unsigned
+		// x-amz-* headers (an unsigned x-amz-copy-source could otherwise turn a
+		// PUT grant into a server-side copy).
+		if testCase.accessKey != "" && testCase.secretKey != "" {
+			if err = signRequestV4(req, testCase.accessKey, testCase.secretKey); err != nil {
+				t.Fatalf("Test %d: Failed to re-sign HTTP request for copy Object: <ERROR> %v", i, err)
+			}
+		}
 		// Since `apiRouter` satisfies `http.Handler` it has a ServeHTTP to execute the logic of the handler.
 		// Call the ServeHTTP to execute the handler, `func (api objectAPIHandlers) CopyObjectHandler` handles the request.
 		apiRouter.ServeHTTP(rec, req)

cmd/object-ssec-zero-byte_test.go

@@ -165,6 +165,11 @@ func testAPIZeroByteSSECAuthenticatesKey(obj ObjectLayer, instanceType, bucketNa
 		t.Fatal(err)
 	}
 	req.Header.Set(xhttp.AmzCopySource, SlashSeparator+pathJoin(bucketName, object))
+	// Re-sign so x-amz-copy-source is covered by the signature, as real S3
+	// clients send it; the verifier rejects unsigned x-amz-* headers.
+	if err = signRequestV4(req, credentials.AccessKey, credentials.SecretKey); err != nil {
+		t.Fatalf("%s: failed to re-sign UploadPartCopy request: %v", instanceType, err)
+	}
 	rec = httptest.NewRecorder()
 	apiRouter.ServeHTTP(rec, req)
 	if rec.Code != http.StatusForbidden {

cmd/signature-v4-utils.go

@@ -264,14 +264,49 @@ func signV4TrimAll(input string) string {
 	return strings.Join(strings.Fields(input), " ")
 }
 
-// checkMetaHeaders will check if the metadata from header/url is the same with the one from signed headers
-func checkMetaHeaders(signedHeadersMap http.Header, r *http.Request) APIErrorCode {
-	// check values from http header
-	for k, val := range r.Header {
-		if stringsHasPrefixFold(k, "X-Amz-Meta-") {
-			if signedHeadersMap.Get(k) == val[0] {
-				continue
-			}
+// checkUnsignedHeaders rejects any x-amz-* request header that is not covered by
+// the SigV4 signed-headers list. AWS S3 requires every x-amz-* header to be
+// signed and returns AccessDenied ("There were headers present in the request
+// which were not signed") otherwise. Enforcing the same here prevents an
+// unsigned x-amz-* header (for example x-amz-copy-source) from changing the
+// semantics of an already-signed or presigned request: without this check a
+// presigned PUT grant could be turned into a server-side copy that reads any
+// object the signing key can reach.
+//
+// Only headers actually sent by the client are inspected. Server-synthesized
+// x-amz-* headers (e.g. x-amz-tagging derived from a request body, or the
+// post-verification x-amz-signature-age scratch header) are set after signature
+// verification and therefore never reach this walk.
+func checkUnsignedHeaders(signedHeadersMap http.Header, r *http.Request) APIErrorCode {
+	// check headers that arrived on the request
+	for k := range r.Header {
+		if !stringsHasPrefixFold(k, "X-Amz-") {
+			continue
+		}
+		// X-Amz-Content-Sha256 carries the payload hash, not an operation or
+		// authorization input, and is handled specially: for presigned requests
+		// it is read from the query string (getContentSha256Cksum) and any
+		// header copy is ignored, while for signed requests it is bound into the
+		// string-to-sign as the payload hash, so a tampered value fails
+		// signature verification regardless of the signed-headers list. Some
+		// clients send it as an unsigned header, so exempt it to preserve
+		// compatibility without weakening the operation-header protection.
+		if strings.EqualFold(k, xhttp.AmzContentSha256) {
+			continue
+		}
+		// X-Amz-Signature-Age is an internal scratch header written by the
+		// presigned verifier itself, after this check, purely so bucket-policy
+		// evaluation can expose s3:signatureAge. It is never sent or signed by a
+		// client, and exempting it keeps signature verification idempotent when
+		// the same request is verified more than once.
+		if strings.EqualFold(k, xhttp.AmzSignatureAge) {
+			continue
+		}
+		// The header must be a member of the signed-headers list. Testing
+		// membership (not value equality) is essential: an unsigned header whose
+		// first value is empty would otherwise compare equal to the empty string
+		// returned for an absent key and slip through.
+		if _, ok := signedHeadersMap[http.CanonicalHeaderKey(k)]; !ok {
 			return ErrUnsignedHeaders
 		}
 	}

cmd/signature-v4-utils_test.go

@@ -363,8 +363,8 @@ func TestGetContentSha256Cksum(t *testing.T) {
 	}
 }
 
-// Test TestCheckMetaHeaders tests the logic of checkMetaHeaders() function
-func TestCheckMetaHeaders(t *testing.T) {
+// Test TestCheckUnsignedHeaders tests the logic of checkUnsignedHeaders() function
+func TestCheckUnsignedHeaders(t *testing.T) {
 	signedHeadersMap := map[string][]string{
 		"X-Amz-Meta-Test":      {"test"},
 		"X-Amz-Meta-Extension": {"png"},
@@ -384,7 +384,7 @@ func TestCheckMetaHeaders(t *testing.T) {
 	inputHeader.Set("X-Amz-Meta-Extension", expectedMetaExtension)
 	inputHeader.Set("X-Amz-Meta-Name", expectedMetaName)
 	// calling the function being tested.
-	errCode := checkMetaHeaders(signedHeadersMap, r)
+	errCode := checkUnsignedHeaders(signedHeadersMap, r)
 	if errCode != ErrNone {
 		t.Fatalf("Expected the APIErrorCode to be %d, but got %d", ErrNone, errCode)
 	}
@@ -392,7 +392,7 @@ func TestCheckMetaHeaders(t *testing.T) {
 	// Add new metadata in inputHeader
 	inputHeader.Set("X-Amz-Meta-Clone", "fail")
 	// calling the function being tested.
-	errCode = checkMetaHeaders(signedHeadersMap, r)
+	errCode = checkUnsignedHeaders(signedHeadersMap, r)
 	if errCode != ErrUnsignedHeaders {
 		t.Fatalf("Expected the APIErrorCode to be %d, but got %d", ErrUnsignedHeaders, errCode)
 	}
@@ -400,7 +400,7 @@ func TestCheckMetaHeaders(t *testing.T) {
 	// Delete extra metadata from header to don't affect other test
 	inputHeader.Del("X-Amz-Meta-Clone")
 	// calling the function being tested.
-	errCode = checkMetaHeaders(signedHeadersMap, r)
+	errCode = checkUnsignedHeaders(signedHeadersMap, r)
 	if errCode != ErrNone {
 		t.Fatalf("Expected the APIErrorCode to be %d, but got %d", ErrNone, errCode)
 	}
@@ -413,8 +413,71 @@ func TestCheckMetaHeaders(t *testing.T) {
 
 	r.ParseForm()
 	// calling the function being tested.
-	errCode = checkMetaHeaders(signedHeadersMap, r)
+	errCode = checkUnsignedHeaders(signedHeadersMap, r)
 	if errCode != ErrNone {
 		t.Fatalf("Expected the APIErrorCode to be %d, but got %d", ErrNone, errCode)
 	}
+
+	// Regression for the unsigned x-amz-copy-source coverage gap: an x-amz-*
+	// header outside the signed-headers list (here x-amz-copy-source, which the
+	// router uses to select CopyObjectHandler) must be rejected. Previously only
+	// x-amz-meta-* headers were inspected, so this header slipped through and a
+	// presigned/authorized PUT could be turned into a server-side copy.
+	r, err = http.NewRequest(http.MethodPut, "http://play.min.io:9000", nil)
+	if err != nil {
+		t.Fatal("Unable to create http.Request :", err)
+	}
+	r.Header.Set("X-Amz-Copy-Source", "/src/secret.txt")
+	if errCode = checkUnsignedHeaders(signedHeadersMap, r); errCode != ErrUnsignedHeaders {
+		t.Fatalf("unsigned x-amz-copy-source: expected %d, got %d", ErrUnsignedHeaders, errCode)
+	}
+
+	// When the same header is part of the signed-headers list with a matching
+	// value it is allowed through, exactly as for x-amz-meta-*.
+	signedWithCopy := http.Header{}
+	for k, v := range signedHeadersMap {
+		signedWithCopy[k] = v
+	}
+	signedWithCopy.Set("X-Amz-Copy-Source", "/src/secret.txt")
+	if errCode = checkUnsignedHeaders(signedWithCopy, r); errCode != ErrNone {
+		t.Fatalf("signed x-amz-copy-source: expected %d, got %d", ErrNone, errCode)
+	}
+
+	// Membership, not value equality: an unsigned x-amz-* header whose first
+	// value is empty must still be rejected. A value-equality check would
+	// compare "" against the empty string returned for an absent signed header
+	// and wrongly let it through, so a multi-value header like
+	// {"", "/src/secret.txt"} could smuggle an unsigned copy-source.
+	r, err = http.NewRequest(http.MethodPut, "http://play.min.io:9000", nil)
+	if err != nil {
+		t.Fatal("Unable to create http.Request :", err)
+	}
+	r.Header["X-Amz-Copy-Source"] = []string{"", "/src/secret.txt"}
+	if errCode = checkUnsignedHeaders(signedHeadersMap, r); errCode != ErrUnsignedHeaders {
+		t.Fatalf("empty-first unsigned x-amz-copy-source: expected %d, got %d", ErrUnsignedHeaders, errCode)
+	}
+
+	// X-Amz-Content-Sha256 is exempt: it carries the payload hash (handled from
+	// the query for presigned and bound into the string-to-sign for signed
+	// requests), so it is allowed even when it is not in the signed-headers map.
+	r, err = http.NewRequest(http.MethodPut, "http://play.min.io:9000", nil)
+	if err != nil {
+		t.Fatal("Unable to create http.Request :", err)
+	}
+	r.Header.Set(xhttp.AmzContentSha256, unsignedPayload)
+	if errCode = checkUnsignedHeaders(signedHeadersMap, r); errCode != ErrNone {
+		t.Fatalf("unsigned x-amz-content-sha256 must be exempt: expected %d, got %d", ErrNone, errCode)
+	}
+
+	// X-Amz-Signature-Age is the presigned verifier's own scratch header,
+	// written after this check. Exempting it keeps verification idempotent when
+	// the same request object is verified more than once.
+	r, err = http.NewRequest(http.MethodPut, "http://play.min.io:9000", nil)
+	if err != nil {
+		t.Fatal("Unable to create http.Request :", err)
+	}
+	r.Header.Set(xhttp.AmzSignatureAge, "1234")
+	if errCode = checkUnsignedHeaders(signedHeadersMap, r); errCode != ErrNone {
+		t.Fatalf("internal x-amz-signature-age must be exempt: expected %d, got %d", ErrNone, errCode)
+	}
 }

cmd/signature-v4.go

@@ -229,10 +229,11 @@ func doesPresignedSignatureMatch(hashedPayload string, r *http.Request, region s
 		return errCode
 	}
 
-	// Check if the metadata headers are equal with signedheaders
-	errMetaCode := checkMetaHeaders(extractedSignedHeaders, r)
-	if errMetaCode != ErrNone {
-		return errMetaCode
+	// Reject any x-amz-* header that the client did not sign. Without this an
+	// unsigned header (e.g. x-amz-copy-source) could alter the request that the
+	// presigned URL actually authorized.
+	if errUnsigned := checkUnsignedHeaders(extractedSignedHeaders, r); errUnsigned != ErrNone {
+		return errUnsigned
 	}
 
 	// If the host which signed the request is slightly ahead in time (by less than globalMaxSkewTime) the
@@ -335,7 +336,7 @@ func doesPresignedSignatureMatch(hashedPayload string, r *http.Request, region s
 		return ErrSignatureDoesNotMatch
 	}
 
-	r.Header.Set("x-amz-signature-age", strconv.FormatInt(UTCNow().Sub(pSignValues.Date).Milliseconds(), 10))
+	r.Header.Set(xhttp.AmzSignatureAge, strconv.FormatInt(UTCNow().Sub(pSignValues.Date).Milliseconds(), 10))
 
 	return ErrNone
 }
@@ -363,6 +364,14 @@ func doesSignatureMatch(hashedPayload string, r *http.Request, region string, st
 		return errCode
 	}
 
+	// Reject any x-amz-* header that the client did not sign. The Authorization
+	// header path shares extractSignedHeaders with the presigned path but, prior
+	// to this, never inspected the headers that actually arrived, so an unsigned
+	// x-amz-copy-source could redirect a signed PUT into a server-side copy.
+	if errUnsigned := checkUnsignedHeaders(extractedSignedHeaders, r); errUnsigned != ErrNone {
+		return errUnsigned
+	}
+
 	cred, _, s3Err := checkKeyValid(r, signV4Values.Credential.accessKey)
 	if s3Err != ErrNone {
 		return s3Err

cmd/signature-v4_test.go

@@ -25,6 +25,8 @@ import (
 	"os"
 	"testing"
 	"time"
+
+	xhttp "github.com/minio/minio/internal/http"
 )
 
 func niceError(code APIErrorCode) string {
@@ -313,3 +315,42 @@ func TestDoesPresignedSignatureMatch(t *testing.T) {
 		}
 	}
 }
+
+// TestPresignedVerifyIdempotent guards against a regression where verifying the
+// same presigned request twice began to fail. doesPresignedSignatureMatch
+// writes an internal x-amz-signature-age header after validating the signature;
+// the unsigned-header check must exempt that scratch header (and an unsigned
+// x-amz-content-sha256 the client may carry) so a second verification of the
+// same *http.Request still succeeds.
+func TestPresignedVerifyIdempotent(t *testing.T) {
+	ctx, cancel := context.WithCancel(t.Context())
+	defer cancel()
+
+	obj, fsDir, err := prepareFS(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(fsDir)
+	if err = newTestConfig(globalMinioDefaultRegion, obj); err != nil {
+		t.Fatal(err)
+	}
+
+	req, err := newTestRequest(http.MethodGet, "http://127.0.0.1:9000/bucket/object", 0, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err = preSignV4(req, globalActiveCred.AccessKey, globalActiveCred.SecretKey, int64(10*60)); err != nil {
+		t.Fatal(err)
+	}
+	if err = req.ParseForm(); err != nil {
+		t.Fatal(err)
+	}
+
+	if got := reqSignatureV4Verify(req, globalSite.Region(), serviceS3); got != ErrNone {
+		t.Fatalf("first verification: expected ErrNone, got %s", niceError(got))
+	}
+	if got := reqSignatureV4Verify(req, globalSite.Region(), serviceS3); got != ErrNone {
+		t.Fatalf("second verification of the same request: expected ErrNone, got %s (x-amz-signature-age=%q)",
+			niceError(got), req.Header.Get(xhttp.AmzSignatureAge))
+	}
+}

internal/http/headers.go

@@ -132,6 +132,11 @@ const (
 	AmzMaxParts             = "X-Amz-Max-Parts"
 	AmzPartNumberMarker     = "X-Amz-Part-Number-Marker"
 
+	// AmzSignatureAge is an internal scratch header the presigned verifier
+	// writes after validating the signature so that bucket-policy evaluation can
+	// expose s3:signatureAge. It is never sent or signed by a client.
+	AmzSignatureAge = "X-Amz-Signature-Age"
+
 	// Constants used for GetObjectAttributes and GetObjectVersionAttributes
 	AmzObjectAttributes = "X-Amz-Object-Attributes"