AI Skills

name: security-express description: |- Review Express.js security audit patterns for middleware and routes. Use for auditing Helmet.js, CORS, body-parser limits, and auth middleware. Use proactively when reviewing Express.js apps. Examples:

user: "Secure my Express app" → add Helmet.js and disable x-powered-by
user: "Check Express CORS config" → verify origin allowlists and credentials
user: "Review Express auth middleware" → check route order and coverage
user: "Scan for Express path traversal" → verify path normalization and validation
user: "Audit Express session config" → check secure, httpOnly, and sameSite flags

Security audit patterns for Express.js applications covering essential security middleware, CORS configuration, auth patterns, and common vulnerabilities.

</overview> <rules>

Essential Security Middleware

Helmet.js (Security Headers)

// Missing security headers - MUST NOT do this
const app = express();

// MUST use Helmet
const helmet = require('helmet');
app.use(helmet());

MUST check if Helmet is installed and used. It sets:

Content-Security-Policy
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
Strict-Transport-Security
And more...

Disable X-Powered-By

// Default - header reveals framework
const app = express();

// MUST disable fingerprinting
app.disable('x-powered-by');
// or: app.set('x-powered-by', false);

CORS Configuration

// CRITICAL: Allow all origins - MUST NOT do this
app.use(cors());
app.use(cors({ origin: '*' }));

// HIGH: Reflect origin with credentials - MUST NOT do this
app.use(cors({ 
  origin: true,  // Reflects any origin!
  credentials: true 
}));

// MUST use explicit allowlist
app.use(cors({
  origin: ['https://app.example.com', 'https://admin.example.com'],
  credentials: true,
}));

// MAY use function for dynamic validation
app.use(cors({
  origin: (origin, callback) => {
    const allowed = ['https://app.example.com'];
    if (!origin || allowed.includes(origin)) {
      callback(null, true);
    } else {
      callback(new Error('Not allowed by CORS'));
    }
  },
  credentials: true,
}));

Body Parser Limits

// No limit (DoS risk) - MUST NOT do this
app.use(express.json());

// MUST set reasonable limits
app.use(express.json({ limit: '100kb' }));
app.use(express.urlencoded({ extended: true, limit: '100kb' }));

</rules> <vulnerabilities>

Auth Middleware Patterns

Missing Auth on Routes

// No auth on admin routes - MUST NOT do this
app.get('/api/admin/users', async (req, res) => {
  res.json(await User.find());
});

// MUST apply auth middleware
app.get('/api/admin/users', requireAuth, requireAdmin, async (req, res) => {
  res.json(await User.find());
});

Middleware Order Matters

// Wrong order - static files before auth - MUST NOT do this
app.use(express.static('uploads')); // Exposed!
app.use(requireAuth);

// MUST place auth before protected static files
app.use('/public', express.static('public')); // Intentionally public
app.use(requireAuth);
app.use('/uploads', express.static('uploads')); // Now protected

Router-Level Auth Gaps

// SHOULD check: Is auth applied to all routes in admin router?
const adminRouter = express.Router();
adminRouter.use(requireAuth); // Applied to all routes below
adminRouter.get('/users', getUsers);
adminRouter.delete('/users/:id', deleteUser);

// Watch for routes defined BEFORE the middleware
const apiRouter = express.Router();
apiRouter.get('/health', getHealth); // No auth (intentional?)
apiRouter.use(requireAuth);
apiRouter.get('/users', getUsers); // Has auth

Common Vulnerabilities

SQL/NoSQL Injection

// String interpolation - MUST NOT do this
const user = await db.query(`SELECT * FROM users WHERE id = ${req.params.id}`);

// MUST use parameterized query
const user = await db.query('SELECT * FROM users WHERE id = $1', [req.params.id]);

// MongoDB injection risk
const user = await User.findOne({ email: req.body.email }); // If email is { $gt: "" }

// MUST validate input type
if (typeof req.body.email !== 'string') return res.status(400).json({ error: 'Invalid email' });

Path Traversal

// User-controlled path - MUST NOT do this
app.get('/files/:filename', (req, res) => {
  res.sendFile(`./uploads/${req.params.filename}`); // ../../etc/passwd
});

// MUST validate and normalize
const path = require('path');
app.get('/files/:filename', (req, res) => {
  const filename = path.basename(req.params.filename);
  const filepath = path.join(__dirname, 'uploads', filename);
  if (!filepath.startsWith(path.join(__dirname, 'uploads'))) {
    return res.status(400).json({ error: 'Invalid path' });
  }
  res.sendFile(filepath);
});

Error Handling

// Stack traces in production - MUST NOT do this
app.use((err, req, res, next) => {
  res.status(500).json({ error: err.stack }); // Leaks internals
});

// MUST use safe error handler
app.use((err, req, res, next) => {
  console.error(err); // Log for debugging
  res.status(500).json({ error: 'Internal server error' });
});

Session Security

// Insecure session config - MUST NOT do this
app.use(session({
  secret: 'keyboard cat', // Hardcoded!
  cookie: { secure: false }, // No HTTPS requirement
}));

// MUST use secure config
app.use(session({
  secret: process.env.SESSION_SECRET,
  resave: false,
  saveUninitialized: false,
  cookie: {
    secure: true, // HTTPS only
    httpOnly: true, // No JS access
    sameSite: 'strict', // CSRF protection
    maxAge: 1000 * 60 * 60 * 24, // 24 hours
  },
}));

Rate Limiting

SHOULD check for rate limiting on auth routes:

const rateLimit = require('express-rate-limit');

const authLimiter = rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 5, // 5 attempts
  message: 'Too many login attempts',
});

app.post('/api/login', authLimiter, loginHandler);
app.post('/api/register', authLimiter, registerHandler);
app.post('/api/forgot-password', authLimiter, forgotPasswordHandler);

</vulnerabilities> <commands>

Quick Audit Commands

# Check if Helmet is used
rg -n 'helmet\\(' . -g "*.js" -g "*.ts"

# Check if x-powered-by is disabled
rg -n "x-powered-by" . -g "*.js" -g "*.ts"

# Check for helmet
rg "helmet" package.json
rg "require\\(['\"]helmet" .
rg "from ['\"]helmet" .

# Find CORS config
rg "cors\\(" . -g "*.js" -g "*.ts" -A 5

# Find routes without auth middleware
rg "app\\.(get|post|put|delete|patch)\\(" . -A 1 | grep -v "require.*[Aa]uth"

# Find string interpolation in queries
rg "(query|find|findOne|exec).*\\`" . -g "*.js" -g "*.ts"

# Check session config
rg "session\\(" . -A 10

</commands> <checklist>

Hardening Checklist

</checklist>

security-express

SKILL.md