Csrf Protection Skill for Claude
Implements CSRF protection using synchronizer tokens, double-submit cookies, and SameSite attributes. Use when securing web forms, protecting state-changing endpoints, or implementing defense-in-depth authentication.
Install
one command, takes just this skill from the repositorynpx skills add https://github.com/secondsky/claude-skills --skill csrf-protectionThe instruction itself
8 sections, as written by the authorCSRF 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
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How to use it
Take secondsky/csrf-protection from the repository into ~/.claude/skills for personal
use, or into .claude/skills inside a project.
The agent identifies a skill by the name field in its header. Two skills with the
same name cannot sit side by side — one of them will be ignored.