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A wss:// WebSocket enters a single TLS session from several threads: the read path, the application's send()/close(), and the heartbeat ping thread. The existing write_mutex_ only serializes writers, so a reader's SSL_read and a writer's SSL_write (plus the SSL_peek in is_peer_closed() on the write path) run concurrently on the same session. OpenSSL and the other backends forbid concurrent access to one session, so this corrupts the record layer: messages are silently dropped, and under ASan it shows up as a heap-buffer-overflow. It affects wss:// only; plain ws:// is unaffected because the kernel allows concurrent recv()/send() on a socket. Route wss:// through a new WebSocketSSLStream that serializes every TLS call with one per-stream mutex. The socket is kept non-blocking for the stream's lifetime and each read()/write() performs a single non-blocking TLS call under the lock, then waits for readiness with select() outside the lock. The lock is therefore held only for CPU-bound work, so a reader blocked waiting for data never stalls a concurrent sender. Because the socket is non-blocking, a TLS call can stop needing either direction, so read() also waits for writability on WantWrite and write() waits for readability on WantRead. A read that shares its session with the send path has to flush pending output before it can decrypt more input, and Mbed TLS surfaces this on every mbedtls_ssl_read(). The read timeouts are atomic since WebSocket::close() shortens them from the closing thread while the receive thread is inside wait_readable(). SSLSocketStream is left untouched, so ordinary HTTP/HTTPS keeps its exact code path and performance. The heartbeat ping thread also stays, so timer-driven pings keep working as before. Add test_websocket_thread_safety.cc, which drives send/close/heartbeat against a concurrent reader over wss://. Built with ASan in CI, a regression surfaces as a heap-buffer-overflow.