Authentication in WebSocket connections in Express - Performance & Optimization

Performance: Authentication in WebSocket connections

MEDIUM IMPACT

This affects the initial connection setup time and ongoing message handling responsiveness in WebSocket communication.

Authenticate users on WebSocket connection

Express

const WebSocket = require('ws');
const wss = new WebSocket.Server({
  port: 8080,
  verifyClient: (info, done) => {
    const token = extractTokenFromInfo(info);
    if (authenticate(token)) {
      done(true);
    } else {
      done(false, 401, 'Unauthorized');
    }
  }
});
wss.on('connection', function connection(ws) {
  // User is authenticated once during handshake
  ws.on('message', function incoming(message) {
    // Process message directly
  });
});

Authentication happens once during handshake, reducing CPU load and improving message responsiveness.

📈 Performance GainSingle authentication per connection, reducing INP and improving user experience.

Authenticate users on WebSocket connection

Express

const WebSocket = require('ws');
const wss = new WebSocket.Server({ port: 8080 });
wss.on('connection', function connection(ws) {
  ws.on('message', function incoming(message) {
    // Authenticate user on every message
    if (!authenticate(message.token)) {
      ws.close();
      return;
    }
    // Process message
  });
});

Authenticating on every message causes repeated CPU work and delays message processing.

📉 Performance CostBlocks message handling, increasing INP and causing slower response to user input.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Authenticate on every message	N/A (WebSocket messages)	N/A	N/A	[X] Bad
Authenticate once during handshake	N/A (WebSocket messages)	N/A	N/A	[OK] Good

Rendering Pipeline

Authentication during WebSocket handshake happens before connection is established, avoiding delays during message processing.

→Connection Setup

→Message Handling

⚠️ BottleneckMessage Handling stage if authentication is repeated per message

Core Web Vital Affected

INP

This affects the initial connection setup time and ongoing message handling responsiveness in WebSocket communication.

Optimization Tips

1Authenticate WebSocket connections once during the handshake to avoid repeated CPU work.

2Avoid authenticating on every message to keep message handling fast and responsive.

3Use DevTools Network panel to verify authentication happens only during connection setup.

Performance Quiz - 3 Questions

What is the main performance benefit of authenticating WebSocket connections during the handshake instead of on every message?

A Reduces CPU load by authenticating once per connection

B Improves page load speed by delaying authentication

C Increases security by authenticating multiple times

D Reduces network bandwidth by compressing tokens

DevTools: Network

How to check: Open DevTools Network panel, filter by WS connections, inspect the initial handshake request and response headers to verify authentication happens once.

What to look for: Look for authentication tokens or headers in the handshake and absence of repeated authentication in message frames.

Practice

1. In Express with WebSocket, when does authentication usually happen for a WebSocket connection?

easy

A. During the HTTP upgrade request before the WebSocket connection is established

B. After the WebSocket connection is fully established

C. Only when sending the first message over WebSocket

D. Authentication is not needed for WebSocket connections

Solution

1. Step 1: Understand WebSocket handshake

   The WebSocket connection starts with an HTTP upgrade request from client to server.

2. Step 2: Identify when authentication fits

   Authentication must happen during this upgrade request to allow or deny the connection before it opens.

3. Final Answer:

   During the HTTP upgrade request before the WebSocket connection is established -> Option A

4. Quick Check:

   Authentication = HTTP upgrade request [OK]

Hint: Authenticate during upgrade request, not after connection opens [OK]

Common Mistakes:

• Trying to authenticate after WebSocket connection opens
• Assuming authentication is unnecessary for WebSocket
• Authenticating only on first message instead of handshake

2. Which of the following is a correct way to send an authentication token during a WebSocket connection upgrade in Express?

easy

A. Include the token in the WebSocket message after connection opens

B. Send the token as a URL query parameter in the WebSocket URL

C. Send the token in a cookie after the connection is established

D. Send the token in a separate HTTP POST request after connection

Solution

1. Step 1: Review token sending methods during upgrade

   Tokens can be sent in the URL query string or headers during the HTTP upgrade request.

2. Step 2: Identify valid option

   Sending the token as a URL query parameter is a common and valid method to authenticate during upgrade.

3. Final Answer:

   Send the token as a URL query parameter in the WebSocket URL -> Option B

4. Quick Check:

   Token in URL query = valid during upgrade [OK]

Hint: Use URL query or headers to send token during upgrade [OK]

Common Mistakes:

• Sending token only after connection opens
• Using cookies after connection instead of during upgrade
• Sending token in separate HTTP requests unrelated to upgrade

3. Consider this Express WebSocket upgrade handler snippet:

server.on('upgrade', (req, socket, head) => {
  const token = new URL(req.url, 'http://localhost').searchParams.get('token');
  if (token !== 'secret') {
    socket.write('HTTP/1.1 401 Unauthorized\r\n\r\n');
    socket.destroy();
    return;
  }
  wss.handleUpgrade(req, socket, head, ws => {
    wss.emit('connection', ws, req);
  });
});

What happens if the token is incorrect?

medium

A. The server ignores the token and accepts the connection

B. The WebSocket connection is established but messages are blocked

C. The server throws an error and crashes

D. The socket connection is closed immediately with 401 Unauthorized

Solution

1. Step 1: Analyze token check and socket handling

   If the token is not 'secret', the code writes a 401 Unauthorized response and destroys the socket immediately.

2. Step 2: Understand socket.destroy effect

   Destroying the socket closes the connection before WebSocket handshake completes, rejecting the client.

3. Final Answer:

   The socket connection is closed immediately with 401 Unauthorized -> Option D

4. Quick Check:

   Invalid token = socket closed with 401 [OK]

Hint: Invalid token triggers socket destroy with 401 response [OK]

Common Mistakes:

• Assuming connection is accepted but blocked later
• Thinking server crashes on invalid token
• Ignoring socket.destroy effect

4. Given this Express WebSocket upgrade code snippet:

server.on('upgrade', (req, socket, head) => {
  const token = req.headers['sec-websocket-protocol'];
  if (!token || token !== 'valid-token') {
    socket.end();
    return;
  }
  wss.handleUpgrade(req, socket, head, ws => {
    wss.emit('connection', ws, req);
  });
});

What is the main issue with this authentication approach?

medium

A. Using 'sec-websocket-protocol' header for token is incorrect and may cause handshake failure

B. Calling socket.end() instead of socket.destroy() leaks resources

C. Not checking token case-insensitively causes false negatives

D. Emitting 'connection' event before handleUpgrade causes errors

Solution

1. Step 1: Understand 'sec-websocket-protocol' header usage

   This header is meant for subprotocol negotiation, not for authentication tokens.

2. Step 2: Recognize handshake impact

   Misusing this header can cause handshake failure or client rejection because clients expect specific values here.

3. Final Answer:

   Using 'sec-websocket-protocol' header for token is incorrect and may cause handshake failure -> Option A

4. Quick Check:

   Wrong header for token breaks handshake [OK]

Hint: Use proper headers or URL query for tokens, not subprotocol header [OK]

Common Mistakes:

• Assuming any header can carry token safely
• Ignoring WebSocket protocol header purpose
• Confusing socket.end() and socket.destroy() effects

5. You want to implement token-based authentication for WebSocket connections in Express. Which approach correctly combines token extraction and connection rejection to secure the WebSocket handshake?

server.on('upgrade', (req, socket, head) => {
  // Extract token
  const token = req.headers['authorization']?.split(' ')[1];
  // Verify token
  if (!token || token !== 'my-secret-token') {
    // Reject connection
    socket.write('HTTP/1.1 401 Unauthorized\r\n\r\n');
    socket.destroy();
    return;
  }
  // Accept connection
  wss.handleUpgrade(req, socket, head, ws => {
    wss.emit('connection', ws, req);
  });
});

hard

A. socket.destroy() should be called after wss.handleUpgrade to reject connection

B. Token extraction from Authorization header is invalid; should use cookies instead

C. This approach correctly extracts token from Authorization header and rejects unauthorized connections during upgrade

D. Rejecting connection by writing 401 is unnecessary; just call socket.end()

Solution

1. Step 1: Check token extraction method

   The code extracts the token from the Authorization header using standard Bearer token format.

2. Step 2: Verify rejection method

   If token is invalid, it writes a 401 Unauthorized response and destroys the socket before upgrade completes, properly rejecting connection.

3. Step 3: Confirm acceptance flow

   If token is valid, it calls handleUpgrade and emits connection event to accept the WebSocket connection.

4. Final Answer:

   This approach correctly extracts token from Authorization header and rejects unauthorized connections during upgrade -> Option C

5. Quick Check:

   Authorization header + socket.destroy() = secure upgrade [OK]

Hint: Extract token from Authorization header, reject with 401 and destroy socket [OK]

Common Mistakes:

• Using cookies instead of headers for token during upgrade
• Calling socket.destroy() after upgrade instead of before
• Not sending 401 response before closing socket