diff --git a/gumble/gumble/udp15_test.go b/gumble/gumble/udp15_test.go
index e51adde..e38599f 100644
--- a/gumble/gumble/udp15_test.go
+++ b/gumble/gumble/udp15_test.go
@@ -128,6 +128,31 @@ func TestCryptState15RoundTrip(t *testing.T) {
 	t.Logf("Server IV after 2 decrypts: %s", hex.EncodeToString(csServer.decryptIV[:]))
 }
 
+// Regression coverage for the native UDP replay window: a captured packet
+// must not be accepted twice after its IV byte has entered history.
+func TestCryptState15RejectsReplay(t *testing.T) {
+	key := mustDecodeHex("93360b0f86a926c4561563469026eb94")
+	clientNonce := mustDecodeHex("d4e53c00a2f6512a61cbe8540eba6314")
+	serverNonce := mustDecodeHex("1463352a4d2375a2695ae0800b22d71d")
+	out, in := &cryptState15{}, &cryptState15{}
+	if err := out.setup15(key, clientNonce, serverNonce); err != nil {
+		t.Fatal(err)
+	}
+	if err := in.setup15(key, serverNonce, clientNonce); err != nil {
+		t.Fatal(err)
+	}
+	packet, err := out.encrypt15([]byte("captured frame"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := in.decrypt15(packet); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := in.decrypt15(packet); err == nil {
+		t.Fatal("replayed UDP packet was accepted")
+	}
+}
+
 // Test protobuf encode/decode round-trip.
 func TestUDPAudioProtobuf(t *testing.T) {
 	tests := []struct {