csrf-protection

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

```javascript
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