diff --git a/cmd/auth-handler.go b/cmd/auth-handler.go
index 884fa9c83..fa35d9058 100644
--- a/cmd/auth-handler.go
+++ b/cmd/auth-handler.go
@@ -632,18 +632,11 @@ func isReqAuthenticated(ctx context.Context, r *http.Request, region string, sty
 		return ErrInvalidDigest
 	}
 
-	// Extract either 'X-Amz-Content-Sha256' header or 'X-Amz-Content-Sha256' query parameter (if V4 presigned)
-	// Do not verify 'X-Amz-Content-Sha256' if skipSHA256.
+	// Honor the selected header/query checksum, including the header fallback
+	// for a presigned request. STS separately hashes its body for its signature.
 	var contentSHA256 []byte
-	if skipSHA256 := skipContentSha256Cksum(r); !skipSHA256 && isRequestPresignedSignatureV4(r) {
-		if sha256Sum, ok := r.Form[xhttp.AmzContentSha256]; ok && len(sha256Sum) > 0 {
-			contentSHA256, err = hex.DecodeString(sha256Sum[0])
-			if err != nil {
-				return ErrContentSHA256Mismatch
-			}
-		}
-	} else if _, ok := r.Header[xhttp.AmzContentSha256]; !skipSHA256 && ok {
-		contentSHA256, err = hex.DecodeString(r.Header.Get(xhttp.AmzContentSha256))
+	if !skipContentSha256Cksum(r) {
+		contentSHA256, err = hex.DecodeString(getContentSha256Cksum(r, serviceS3))
 		if err != nil || len(contentSHA256) == 0 {
 			return ErrContentSHA256Mismatch
 		}
diff --git a/cmd/presigned-payload_test.go b/cmd/presigned-payload_test.go
new file mode 100644
index 000000000..ef39a0a84
--- /dev/null
+++ b/cmd/presigned-payload_test.go
@@ -0,0 +1,114 @@
+// Copyright (c) 2026 PGSTY
+// SPDX-License-Identifier: AGPL-3.0-only
+
+package cmd
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/minio/minio/internal/auth"
+	xhttp "github.com/minio/minio/internal/http"
+)
+
+func TestPresignedHeaderPayloadHashIsEnforced(t *testing.T) {
+	setupSignatureBoundaryTest(t)
+	digest := getSHA256Hash([]byte("expected"))
+	for _, tc := range []struct {
+		name, query, header string
+		unsigned            bool
+	}{
+		{name: "header only", header: digest},
+		{name: "query only", query: digest},
+		{name: "query wins", query: digest, header: getSHA256Hash([]byte("different"))},
+		{name: "unsigned query wins", query: unsignedPayload, header: digest, unsigned: true},
+		{name: "unsigned header", header: unsignedPayload, unsigned: true},
+		{name: "absent", unsigned: true},
+	} {
+		for _, body := range []string{"expected", "modified"} {
+			t.Run(tc.name+"/"+body, func(t *testing.T) {
+				r := httptest.NewRequest(http.MethodPut, "http://minio.local/bucket/object", strings.NewReader(body))
+				if tc.header != "" {
+					r.Header.Set(xhttp.AmzContentSha256, tc.header)
+				}
+				if tc.query != "" {
+					query := r.URL.Query()
+					query.Set(xhttp.AmzContentSha256, tc.query)
+					r.URL.RawQuery = query.Encode()
+				}
+				presignBoundaryRequest(t, r, UTCNow(), []string{"host"}, globalActiveCred)
+				if code := isReqAuthenticated(t.Context(), r, globalSite.Region(), serviceS3); code != ErrNone {
+					t.Fatal(niceError(code))
+				}
+				_, err := io.ReadAll(r.Body)
+				wantMismatch := body == "modified" && !tc.unsigned
+				if wantMismatch {
+					if toAPIErrorCode(t.Context(), err) != ErrContentSHA256Mismatch {
+						t.Errorf("modified signed payload: got %v, want SHA-256 mismatch", err)
+					}
+				} else if err != nil {
+					t.Errorf("valid payload: %v", err)
+				}
+			})
+		}
+	}
+}
+
+func TestAPIPresignedBucketPolicyPayloadHash(t *testing.T) {
+	defer DetectTestLeak(t)()
+	ExecObjectLayerAPITest(ExecObjectLayerAPITestArgs{
+		t: t,
+		objAPITest: func(objectAPI ObjectLayer, instanceType, bucket string, apiRouter http.Handler, credentials auth.Credentials, t *testing.T) {
+			server := httptest.NewServer(apiRouter)
+			defer server.Close()
+			client := server.Client()
+			client.Timeout = 10 * time.Second
+			expected := fmt.Sprintf(`{"Version":"2012-10-17","Statement":[{"Effect":"Allow","Principal":{"AWS":["*"]},"Action":["s3:GetObject"],"Resource":["arn:aws:s3:::%s/*"]}]}`, bucket)
+			modified := strings.Replace(expected, "s3:GetObject", "s3:PutObject", 1)
+			for _, tc := range []struct {
+				name, body string
+				status     int
+			}{
+				{"original", expected, http.StatusNoContent},
+				{"modified", modified, http.StatusBadRequest},
+			} {
+				t.Run(instanceType+"/"+tc.name, func(t *testing.T) {
+					r, err := http.NewRequestWithContext(t.Context(), http.MethodPut, server.URL+"/"+bucket+"?policy=", strings.NewReader(tc.body))
+					if err != nil {
+						t.Fatal(err)
+					}
+					r.Header.Set(xhttp.AmzContentSha256, getSHA256Hash([]byte(expected)))
+					presignBoundaryRequest(t, r, UTCNow(), []string{"host"}, credentials)
+					response, err := client.Do(r)
+					if err != nil {
+						t.Fatal(err)
+					}
+					defer response.Body.Close()
+					body, err := io.ReadAll(response.Body)
+					if err != nil {
+						t.Fatal(err)
+					}
+					if response.StatusCode != tc.status {
+						t.Fatalf("PutBucketPolicy: got %d %s, want %d", response.StatusCode, body, tc.status)
+					}
+					if tc.status == http.StatusBadRequest && !strings.Contains(string(body), "XAmzContentSHA256Mismatch") {
+						t.Fatalf("expected payload checksum rejection, got %s", body)
+					}
+				})
+			}
+			meta, err := globalBucketMetadataSys.Get(bucket)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if strings.Contains(string(meta.PolicyConfigJSON), "s3:PutObject") {
+				t.Fatal("tampered presigned request replaced the bucket policy")
+			}
+		},
+		endpoints: []string{"PutBucketPolicy"},
+	})
+}