security: WS auth via first-message, not query string

Tokens in URL leak through proxy access logs, browser history and
Referer headers. Now: WS opens unauthenticated, client sends
{type:'auth', token} as first message; server responds with
{type:'auth_ok'} and starts normal message processing.
5-second timeout closes any unauthenticated connection.
Frontend queues session join until auth_ok received.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/ws-server.js

@@ -279,27 +279,21 @@ function attach(httpServer) {
   const wss = new WebSocketServer({ server: httpServer, path: '/ws' });
   _wss = wss;
 
-  wss.on('connection', (ws, req) => {
-    /* ── Auth ── */
-    let user = null;
-    try {
-      const url   = new URL(req.url, 'http://localhost');
-      const token = url.searchParams.get('token') || '';
-      user = jwt.verify(token, process.env.JWT_SECRET, { algorithms: ['HS256'] });
-    } catch {
-      ws.close(4001, 'Unauthorized');
-      return;
-    }
-
-    ws.userId   = user.id;
-    ws.userName = user.name || user.email || '';
+  wss.on('connection', (ws) => {
+    ws.userId   = null;
+    ws.userName = '';
     ws.isAlive  = true;
 
     ws.on('pong', () => { ws.isAlive = true; });
 
-    ws._msgCount = 0;
+    ws._msgCount       = 0;
     ws._msgWindowStart = Date.now();
 
+    // Close unauthenticated connections after 5s
+    ws._authTimer = setTimeout(() => {
+      if (!ws.userId) { try { ws.close(4001, 'Auth timeout'); } catch {} }
+    }, 5000);
+
     ws.on('message', raw => {
       // Rate-limit: max 120 messages per second per connection
       const now = Date.now();
@@ -308,13 +302,37 @@ function attach(httpServer) {
 
       let msg;
       try { msg = JSON.parse(raw); } catch { return; }
+
+      // Pre-auth: only {type:'auth', token} accepted
+      if (!ws.userId) {
+        if (msg.type !== 'auth' || !msg.token) {
+          try { ws.close(4001, 'Auth required'); } catch {}
+          return;
+        }
+        try {
+          const user = jwt.verify(msg.token, process.env.JWT_SECRET, { algorithms: ['HS256'] });
+          ws.userId   = user.id;
+          ws.userName = user.name || user.email || '';
+          clearTimeout(ws._authTimer);
+          ws._authTimer = null;
+          try { ws.send(JSON.stringify({ type: 'auth_ok' })); } catch {}
+        } catch {
+          try { ws.close(4001, 'Invalid token'); } catch {}
+        }
+        return;
+      }
+
+      // Post-auth: normal handler
       try { _handleMessage(ws, msg); } catch (e) {
         // swallow — never crash the WS server on bad input
       }
     });
 
     ws.on('error', () => {});
-    ws.on('close', () => { _unregisterUser(ws); });
+    ws.on('close', () => {
+      if (ws._authTimer) { clearTimeout(ws._authTimer); ws._authTimer = null; }
+      if (ws.userId) _unregisterUser(ws);
+    });
   });
 
   /* ── Ping/pong keepalive (30s) ── */

frontend/classroom.html

@@ -4330,12 +4330,11 @@
   function _crWsConnect() {
     if (_crWs && (_crWs.readyState === WebSocket.OPEN || _crWs.readyState === WebSocket.CONNECTING)) return;
     const proto = location.protocol === 'https:' ? 'wss:' : 'ws:';
-    const token = LS.getToken() || '';
-    _crWs = new WebSocket(`${proto}//${location.host}/ws?token=${encodeURIComponent(token)}`);
+    _crWs = new WebSocket(`${proto}//${location.host}/ws`);
     _crWs.onopen = () => {
-      _crWsReady = true;
-      // Register in session so server delivers events via WS instead of SSE
-      if (_sessionId) _crWs.send(JSON.stringify({ type: 'classroom_join', sessionId: _sessionId }));
+      // Authenticate via first message — token never appears in URL or proxy logs
+      const token = LS.getToken() || '';
+      _crWs.send(JSON.stringify({ type: 'auth', token }));
     };
     _crWs.onclose = () => {
       _crWsReady = false;
@@ -4343,9 +4342,17 @@
       if (_sessionId) setTimeout(() => { if (_sessionId) _crWsConnect(); }, 2000);
     };
     _crWs.onerror = () => { _crWsReady = false; };
-    // All classroom server→client events arrive here (WS replaces SSE for classroom)
     _crWs.onmessage = e => {
-      try { handleSSE(JSON.parse(e.data), true); } catch {}
+      let msg;
+      try { msg = JSON.parse(e.data); } catch { return; }
+      if (msg.type === 'auth_ok') {
+        _crWsReady = true;
+        // Register in session so server delivers events via WS instead of SSE
+        if (_sessionId) _crWs.send(JSON.stringify({ type: 'classroom_join', sessionId: _sessionId }));
+        return;
+      }
+      // All other classroom server→client events
+      try { handleSSE(msg, true); } catch {}
     };
   }