flash/src/main/java/dev/relism/flash/websocket/WebSocketSession.java

@@ -133,22 +133,20 @@ public final class WebSocketSession {
     }
 
     /**
-     * Sends a CLOSE frame exactly once.
+     * Sends a CLOSE frame exactly once, masked exactly like any other outgoing frame when this
+     * session is in client mode ({@link #maskOutgoing}) — via {@link #writeFrame}, the same path
+     * {@link #sendText}/{@link #send}/{@link #sendPong} use, rather than hand-writing the 4 bytes
+     * unmasked regardless of role. A client-mode session that skipped masking here was sending an
+     * RFC-6455-invalid CLOSE frame that a strict peer's {@link #readFrame} correctly rejects (the
+     * masking check {@link #readFrame} itself enforces on the receive side).
      * No flush needed — {@code out} is the raw socket OutputStream (not buffered);
      * each write goes directly to the kernel send buffer, and TCP_NODELAY ensures
      * it's transmitted immediately.
      */
     public void close(int code) throws IOException {
         if (!open.compareAndSet(true, false)) return;
-        writeLock.lock();
-        try {
-            out.write(0x88);
-            out.write(0x02);
-            out.write((code >> 8) & 0xFF);
-            out.write(code & 0xFF);
-        } finally {
-            writeLock.unlock();
-        }
+        byte[] payload = {(byte) ((code >> 8) & 0xFF), (byte) (code & 0xFF)};
+        writeFrame(WebSocketFrame.OP_CLOSE, payload, 0, 2);
     }
 
     // ── Session loop internals ─────────────────────────────────────────────

flash/src/test/java/dev/relism/flash/websocket/WebSocketSessionFrameTest.java

@@ -40,4 +40,26 @@ class WebSocketSessionFrameTest {
 
         assertThrows(Exception.class, () -> session.readFrame(frame));
     }
+
+    /**
+     * Regression for the CLOSE frame Pathway's {@code UpstreamWebSocketConnector} (a client-mode
+     * session, {@code maskOutgoing=true}) sends when relaying the client's own CLOSE to a backend:
+     * before this fix, {@link WebSocketSession#close} never masked, so a server-mode peer's own
+     * {@link WebSocketSession#readFrame} (which enforces masking per RFC 6455 §5.1) rejected it
+     * with a {@link WebSocketProtocolException}, and the close was silently lost.
+     */
+    @Test
+    void readFrame_acceptsCloseFrameWrittenByAClientSession() throws Exception {
+        ByteArrayOutputStream wire = new ByteArrayOutputStream();
+        WebSocketSession client = new WebSocketSession(new ByteArrayInputStream(new byte[0]), wire, 64, null, true);
+        client.close(1000);
+
+        WebSocketSession server = new WebSocketSession(new ByteArrayInputStream(wire.toByteArray()), new ByteArrayOutputStream(), 64);
+        WebSocketFrame frame = new WebSocketFrame();
+
+        assertTrue(server.readFrame(frame));
+        assertEquals(WebSocketFrame.OP_CLOSE, frame.opcode());
+        server.closeFromPeer(frame);
+        assertEquals(1000, server.closeCode());
+    }
 }

flash/src/test/java/dev/relism/flash/websocket/WebSocketSessionTest.java

@@ -90,6 +90,26 @@ class WebSocketSessionTest {
         assertEquals((byte) 0xE8, bytes[3]);
     }
 
+    @Test
+    void close_masksWhenActingAsClient() throws Exception {
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        WebSocketSession session = new WebSocketSession(
+                new ByteArrayInputStream(new byte[0]), out, 64, null, true);
+
+        session.close(1000);
+
+        assertFalse(session.isOpen());
+        assertEquals(1000, session.closeCode());
+        byte[] bytes = out.toByteArray();
+        assertEquals((byte) 0x88, bytes[0]); // FIN + CLOSE
+        assertEquals((byte) (0x80 | 2), bytes[1]); // masked bit + length 2
+        // Header is opcode + length + a full 4-byte mask key (bytes 2-5) regardless of payload
+        // length, then the (masked) payload — mirrors sendText_masksWhenActingAsClient above.
+        byte m0 = bytes[2], m1 = bytes[3];
+        assertEquals((byte) (0x03 ^ m0), bytes[6]);
+        assertEquals((byte) (0xE8 ^ m1), bytes[7]);
+    }
+
     @Test
     void closeFromPeer_extractsCloseCode() {
         WebSocketSession session = new WebSocketSession(new ByteArrayInputStream(new byte[0]), new ByteArrayOutputStream(), 64);