CSRF Protection Skill | Agent Skills

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

Agent Skills: CSRF Protection

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