diff --git a/src/dtls12/engine.rs b/src/dtls12/engine.rs
index a0343eb8..6aa23593 100644
--- a/src/dtls12/engine.rs
+++ b/src/dtls12/engine.rs
@@ -1103,10 +1103,14 @@ impl RecordDecrypt for Engine {
         self.peer_encryption_enabled
     }
 
-    fn replay_check_and_update(&mut self, seq: Sequence) -> bool {
+    fn replay_check(&self, seq: Sequence) -> bool {
         // Only epoch 1 (encrypted) records reach here; epoch 0 records are
         // returned early by the DTLS 1.2 incoming parser.
-        self.replay.check_and_update(seq.sequence_number)
+        self.replay.check(seq.sequence_number)
+    }
+
+    fn replay_update(&mut self, seq: Sequence) {
+        self.replay.update(seq.sequence_number);
     }
 
     fn decryption_aad_and_nonce(&self, dtls: &DTLSRecord, buf: &[u8]) -> (Aad, Nonce) {
diff --git a/src/dtls12/incoming.rs b/src/dtls12/incoming.rs
index 26ed3b0b..820c19ba 100644
--- a/src/dtls12/incoming.rs
+++ b/src/dtls12/incoming.rs
@@ -150,9 +150,10 @@ impl Record {
 
         // We need to decrypt the record and redo the parsing.
         let dtls = record.record();
+        let sequence = dtls.sequence;
 
-        // Anti-replay check
-        if !decrypt.replay_check_and_update(dtls.sequence) {
+        // Anti-replay check (read-only, does not update window)
+        if !decrypt.replay_check(sequence) {
             return Ok(None);
         }
 
@@ -180,6 +181,11 @@ impl Record {
             buffer.len()
         };
 
+        // Decryption succeeded — now commit the replay window update.
+        // RFC 6347 §4.1.2.6: "The receive window is updated only if the
+        // MAC verification succeeds."
+        decrypt.replay_update(sequence);
+
         // Update the length of the record.
         buffer[11] = (new_len >> 8) as u8;
         buffer[12] = new_len as u8;
@@ -263,7 +269,8 @@ impl ParsedRecord {
 /// parsing to depend only on the cryptographic operations it actually uses.
 pub trait RecordDecrypt {
     fn is_peer_encryption_enabled(&self) -> bool;
-    fn replay_check_and_update(&mut self, seq: Sequence) -> bool;
+    fn replay_check(&self, seq: Sequence) -> bool;
+    fn replay_update(&mut self, seq: Sequence);
     fn decryption_aad_and_nonce(&self, dtls: &DTLSRecord, buf: &[u8]) -> (Aad, Nonce);
     fn explicit_nonce_len(&self) -> usize;
     fn decrypt_data(
diff --git a/src/dtls13/engine.rs b/src/dtls13/engine.rs
index d6c2e6fc..a973336c 100644
--- a/src/dtls13/engine.rs
+++ b/src/dtls13/engine.rs
@@ -2273,19 +2273,24 @@ impl RecordDecrypt for Engine {
         reconstruct_sequence(seq_bits, expected, bits)
     }
 
-    fn replay_check_and_update(&mut self, seq: Sequence) -> bool {
+    fn replay_check(&self, seq: Sequence) -> bool {
         // Route to the correct per-epoch replay window
-        let accepted = if seq.epoch == 2 {
-            self.hs_replay.check_and_update(seq.sequence_number)
+        if seq.epoch == 2 {
+            self.hs_replay.check(seq.sequence_number)
         } else {
-            match self.app_recv_keys.iter_mut().find(|e| e.epoch == seq.epoch) {
-                Some(entry) => entry.replay.check_and_update(seq.sequence_number),
-                None => return false, // no keys for this epoch
+            match self.app_recv_keys.iter().find(|e| e.epoch == seq.epoch) {
+                Some(entry) => entry.replay.check(seq.sequence_number),
+                None => false, // no keys for this epoch
             }
-        };
+        }
+    }
 
-        if !accepted {
-            return false;
+    fn replay_update(&mut self, seq: Sequence) {
+        // Update the replay window for this epoch
+        if seq.epoch == 2 {
+            self.hs_replay.update(seq.sequence_number);
+        } else if let Some(entry) = self.app_recv_keys.iter_mut().find(|e| e.epoch == seq.epoch) {
+            entry.replay.update(seq.sequence_number);
         }
 
         // Advance expected receive sequence for this epoch
@@ -2304,8 +2309,6 @@ impl RecordDecrypt for Engine {
                 }
             }
         }
-
-        true
     }
 
     fn decrypt_record(
diff --git a/src/dtls13/incoming.rs b/src/dtls13/incoming.rs
index b20559df..79a72cf3 100644
--- a/src/dtls13/incoming.rs
+++ b/src/dtls13/incoming.rs
@@ -245,8 +245,8 @@ impl Record {
             sequence_number: full_seq,
         };
 
-        // Anti-replay check
-        if !decrypt.replay_check_and_update(full_sequence) {
+        // Anti-replay check (read-only, does not update window)
+        if !decrypt.replay_check(full_sequence) {
             return Ok(None);
         }
 
@@ -278,6 +278,11 @@ impl Record {
             buffer.len()
         };
 
+        // Decryption succeeded — now commit the replay window update.
+        // RFC 9147 §4.5.1: "The window MUST NOT be updated due to a received
+        // record until that record has been deprotected successfully."
+        decrypt.replay_update(full_sequence);
+
         // Recover inner content type from DTLSInnerPlaintext
         let decrypted = &buffer[header_end..header_end + new_len];
         let (inner_content_type, content_len) = match recover_inner_content_type(decrypted) {
@@ -395,7 +400,8 @@ pub trait RecordDecrypt {
     fn is_peer_encryption_enabled(&self) -> bool;
     fn resolve_epoch(&self, epoch_bits: u8) -> u16;
     fn resolve_sequence(&self, epoch: u16, seq_bits: u64, s_flag: bool) -> u64;
-    fn replay_check_and_update(&mut self, seq: Sequence) -> bool;
+    fn replay_check(&self, seq: Sequence) -> bool;
+    fn replay_update(&mut self, seq: Sequence);
     fn decrypt_record(
         &mut self,
         header: &[u8],
diff --git a/src/window.rs b/src/window.rs
index d53c7046..66b93e09 100644
--- a/src/window.rs
+++ b/src/window.rs
@@ -16,9 +16,24 @@ impl ReplayWindow {
         Self::default()
     }
 
-    /// Check if the given sequence number is acceptable and update the window state.
-    /// Returns true if fresh/acceptable, false if duplicate/too old.
-    pub fn check_and_update(&mut self, seqno: u64) -> bool {
+    /// Check if the given sequence number is acceptable (not a replay, not too old).
+    /// Read-only: does not modify the window state.
+    pub fn check(&self, seqno: u64) -> bool {
+        if seqno > self.max_seq {
+            true
+        } else {
+            let offset = self.max_seq - seqno;
+            if offset >= 64 {
+                return false; // too old
+            }
+            let mask = 1u64 << offset;
+            (self.window & mask) == 0 // false if duplicate
+        }
+    }
+
+    /// Update the window state to record that `seqno` has been received.
+    /// Must only be called after the record has been authenticated (decrypted successfully).
+    pub fn update(&mut self, seqno: u64) {
         if seqno > self.max_seq {
             let delta = seqno - self.max_seq;
             if delta > 63 {
@@ -29,18 +44,11 @@ impl ReplayWindow {
                 self.window |= 1; // mark newest as seen
             }
             self.max_seq = seqno;
-            true
         } else {
             let offset = self.max_seq - seqno;
-            if offset >= 64 {
-                return false; // too old
-            }
-            let mask = 1u64 << offset;
-            if (self.window & mask) != 0 {
-                return false; // duplicate
+            if offset < 64 {
+                self.window |= 1u64 << offset;
             }
-            self.window |= mask;
-            true
         }
     }
 }
@@ -49,54 +57,87 @@ impl ReplayWindow {
 mod tests {
     use super::*;
 
+    /// Helper: check and update in one step (simulates authenticated record).
+    fn check_and_update(w: &mut ReplayWindow, seqno: u64) -> bool {
+        if w.check(seqno) {
+            w.update(seqno);
+            true
+        } else {
+            false
+        }
+    }
+
     #[test]
     fn accepts_fresh_and_rejects_duplicate() {
         let mut w = ReplayWindow::new();
-        assert!(w.check_and_update(1));
-        assert!(!w.check_and_update(1)); // duplicate
-        assert!(w.check_and_update(2)); // next fresh
+        assert!(check_and_update(&mut w, 1));
+        assert!(!check_and_update(&mut w, 1)); // duplicate
+        assert!(check_and_update(&mut w, 2)); // next fresh
     }
 
     #[test]
     fn accepts_out_of_order_within_window() {
         let mut w = ReplayWindow::new();
-        assert!(w.check_and_update(10)); // establish max=10
-        assert!(w.check_and_update(8)); // unseen within 64
-        assert!(!w.check_and_update(8)); // duplicate now
-        assert!(w.check_and_update(9)); // unseen within 64
+        assert!(check_and_update(&mut w, 10)); // establish max=10
+        assert!(check_and_update(&mut w, 8)); // unseen within 64
+        assert!(!check_and_update(&mut w, 8)); // duplicate now
+        assert!(check_and_update(&mut w, 9)); // unseen within 64
     }
 
     #[test]
     fn rejects_too_old() {
         let mut w = ReplayWindow::new();
-        assert!(w.check_and_update(100));
+        assert!(check_and_update(&mut w, 100));
         // offset = 64 -> too old
-        assert!(!w.check_and_update(36));
+        assert!(!check_and_update(&mut w, 36));
         // offset = 63 -> allowed once
-        assert!(w.check_and_update(37));
+        assert!(check_and_update(&mut w, 37));
     }
 
     #[test]
     fn handles_large_jump_and_window_shift() {
         let mut w = ReplayWindow::new();
-        assert!(w.check_and_update(1));
+        assert!(check_and_update(&mut w, 1));
         // Large forward jump clears the window entirely
-        assert!(w.check_and_update(80));
+        assert!(check_and_update(&mut w, 80));
         // Within window of new max and unseen
-        assert!(w.check_and_update(79));
+        assert!(check_and_update(&mut w, 79));
         // Too old relative to new max
-        assert!(!w.check_and_update(15));
+        assert!(!check_and_update(&mut w, 15));
     }
 
     #[test]
     fn large_jump_does_not_leave_stale_bits() {
         let mut w = ReplayWindow::new();
-        assert!(w.check_and_update(0));
+        assert!(check_and_update(&mut w, 0));
         // Jump of 200 exceeds window size (64). The window must be fully
         // cleared so no stale bits from seq 0 remain.
-        assert!(w.check_and_update(200));
+        assert!(check_and_update(&mut w, 200));
         // seq 137 is within the window (offset = 200 - 137 = 63) and was
         // never seen, so it must be accepted.
-        assert!(w.check_and_update(137));
+        assert!(check_and_update(&mut w, 137));
+    }
+
+    #[test]
+    fn check_does_not_modify_window() {
+        let mut w = ReplayWindow::new();
+        w.update(10);
+        // check alone should not change state
+        assert!(w.check(11));
+        assert!(w.check(11)); // still acceptable because update was never called
+        w.update(11);
+        assert!(!w.check(11)); // now it's a duplicate
+    }
+
+    #[test]
+    fn failed_auth_does_not_advance_window() {
+        let mut w = ReplayWindow::new();
+        w.update(5);
+        // Simulate receiving seq 200 that passes check but fails authentication
+        assert!(w.check(200));
+        // Do NOT call update (authentication failed)
+        // Legitimate packet at seq 6 should still be accepted
+        assert!(w.check(6));
+        w.update(6);
     }
 }