Authentication in WebSocket connections in Express

Introduction

Authentication in WebSocket connections helps verify who is connecting. It keeps your app safe by allowing only trusted users to communicate.

When you want to allow only logged-in users to send and receive messages.

When you need to protect real-time data like chat messages or notifications.

When you want to track user activity securely over a WebSocket.

When your app requires user identity to customize the data sent over WebSocket.

Syntax

Express

const WebSocket = require('ws');
const wss = new WebSocket.Server({ noServer: true });

server.on('upgrade', (request, socket, head) => {
  // Extract token from request headers or URL
  const token = extractToken(request);

  if (isValidToken(token)) {
    wss.handleUpgrade(request, socket, head, (ws) => {
      wss.emit('connection', ws, request);
    });
  } else {
    socket.destroy();
  }
});

Authentication usually happens during the HTTP upgrade request before WebSocket connection is established.

You can extract tokens from headers, cookies, or URL query parameters.

Examples

Using the sec-websocket-protocol header to pass a token for authentication.

Express

const token = request.headers['sec-websocket-protocol'];
if (token === 'my-secret-token') {
  // Accept connection
}

Extracting token from URL query parameters during upgrade request.

Express

const url = new URL(request.url, 'http://localhost');
const token = url.searchParams.get('token');
if (token === 'valid-token') {
  // Accept connection
}

Sample Program

This example shows a simple WebSocket server using Express's HTTP server. It checks a token from the URL query during the upgrade request. If the token is correct, it accepts the connection and sends a welcome message. Otherwise, it rejects the connection.

Express

const http = require('http');
const express = require('express');
const WebSocket = require('ws');

const app = express();
const server = http.createServer(app);
const wss = new WebSocket.Server({ noServer: true });

function isValidToken(token) {
  return token === 'secret123';
}

server.on('upgrade', (request, socket, head) => {
  const url = new URL(request.url, 'http://localhost');
  const token = url.searchParams.get('token');

  if (!isValidToken(token)) {
    socket.write('HTTP/1.1 401 Unauthorized\r\n\r\n');
    socket.destroy();
    return;
  }

  wss.handleUpgrade(request, socket, head, (ws) => {
    wss.emit('connection', ws, request);
  });
});

wss.on('connection', (ws) => {
  ws.send('Welcome! You are authenticated.');
  ws.on('message', (message) => {
    ws.send(`You said: ${message}`);
  });
});

server.listen(8080, () => {
  console.log('Server listening on http://localhost:8080');
});

Important Notes

Always validate tokens securely, for example by checking against a database or using JWT verification.

Do not trust tokens sent from the client without verification.

Use secure WebSocket (wss://) in production to protect data in transit.

Summary

Authentication happens during the HTTP upgrade request before WebSocket connection starts.

Tokens can be sent via headers or URL query parameters for verification.

Reject unauthorized connections by closing the socket early.

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