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'];
if (!token || !crypto.timingSafeEqual(
Buffer.from(token),
Buffer.from(req.session.csrfToken)
)) {
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](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