CSRF Protection

Defend against Cross-Site Request Forgery attacks using multiple protection layers.

Protection Methods

Method	How It Works	Browser Support
Synchronizer Token	Hidden form field validated server-side	All
Double Submit	Cookie + header must match	All
SameSite Cookie	Browser blocks cross-origin requests	Modern

Token-Based Protection (Express)

const crypto = require('crypto');

function generateToken() {
  return crypto.randomBytes(32).toString('hex');
}

// Middleware
app.use((req, res, next) => {
  if (!req.session.csrfToken) {
    req.session.csrfToken = generateToken();
  }
  res.locals.csrfToken = req.session.csrfToken;
  next();
});

// Validation
app.post('*', (req, res, next) => {
  const token = req.body._csrf || req.headers['x-csrf-token'];
  // crypto.timingSafeEqual throws RangeError when buffers differ in length,
  // so check length explicitly first (still constant-time on the equal-length path).
  const csrf = req.session.csrfToken || '';
  if (!token || token.length !== csrf.length) {
    return res.status(403).json({ error: 'Invalid CSRF token' });
  }
  if (!crypto.timingSafeEqual(Buffer.from(token), Buffer.from(csrf))) {
    return res.status(403).json({ error: 'Invalid CSRF token' });
  }
  next();
});

SameSite Cookies

app.use(session({
  cookie: {
    httpOnly: true,
    secure: true,
    sameSite: 'strict', // or 'lax'
    maxAge: 3600000
  }
}));

HTML Form Integration

<form method="POST" action="/transfer">
  <input type="hidden" name="_csrf" value="<%= csrfToken %>">
  <button type="submit">Submit</button>
</form>

Best Practices

• Apply to all state-changing requests (POST, PUT, DELETE)
• Use SameSite=Strict for sensitive cookies
• Validate Origin/Referer headers
• Never use GET for modifications
• Implement token expiration (1 hour typical)
• Combine multiple defense layers

Additional Implementations

See references/python-react.md for:

• Flask-WTF complete CSRF setup
• React hooks for CSRF token management
• Double submit cookie pattern

Common Mistakes

• Assuming authentication prevents CSRF
• Reusing tokens across sessions
• Storing tokens in localStorage
• Missing token expiration