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Fix JWT Vulnerabilities (Weak Signing, None Algo) in Polka

JWT implementation flaws like the 'none' algorithm or weak symmetric keys are classic vectors for session hijacking and privilege escalation. In minimalist Node.js frameworks like Polka, developers often misconfigure middleware, leading to authentication bypasses. This guide covers how to eliminate these vulnerabilities by enforcing strict algorithm validation and robust secret management.

The Vulnerable Pattern

const polka = require('polka');
const jwt = require('jsonwebtoken');
const SECRET = '12345'; // VULNERABILITY: Weak, hardcoded secret

polka()
.use((req, res, next) => {
const authHeader = req.headers[‘authorization’];
if (!authHeader) return next();


try {
  // VULNERABILITY: Explicitly allowing 'none' algorithm and using weak secret
  const token = authHeader.split(' ')[1];
  const decoded = jwt.verify(token, SECRET, { algorithms: ['HS256', 'none'] });
  req.user = decoded;
  next();
} catch (err) {
  res.statusCode = 401;
  res.end('Unauthorized');
}


})
.get(’/’, (req, res) => {
res.end(Welcome ${req.user.id});
})
.listen(3000);

The Secure Implementation

The vulnerable code suffers from Algorithm Confusion and Weak Secret vulnerabilities. By allowing the 'none' algorithm, an attacker can modify the JWT header to `{ "alg": "none" }`, remove the signature entirely, and gain unauthorized access. The weak secret '12345' is trivial to brute-force using tools like Hashcat. The secure implementation fixes this by: 1. Enforcing an asymmetric algorithm (RS256), which ensures only the private key holder can sign tokens while the app uses the public key to verify. 2. Removing 'none' from the allowed algorithms list. 3. Implementing standard claims checks (issuer/audience) to prevent token reuse across different services.

const polka = require('polka');
const jwt = require('jsonwebtoken');
const fs = require('fs');

// SECURE: Use environment variables and asymmetric keys (RS256)
const PUBLIC_KEY = process.env.JWT_PUBLIC_KEY || fs.readFileSync(’./public.pem’);


polka()
.use((req, res, next) => {
const authHeader = req.headers[‘authorization’];
if (!authHeader || !authHeader.startsWith(‘Bearer ’)) return (res.statusCode = 401, res.end());


const token = authHeader.split(' ')[1];

try {
  // SECURE: Enforce RS256, no 'none' allowed, strict algorithm check
  req.user = jwt.verify(token, PUBLIC_KEY, {
    algorithms: ['RS256'],
    issuer: 'auth-service',
    audience: 'app-service'
  });
  next();
} catch (err) {
  res.statusCode = 403;
  res.end('Forbidden: Invalid Token');
}


})
.get(’/’, (req, res) => {
res.end(Secure Access for User: ${req.user.sub});
})
.listen(3000);

System Alert • ID: 2543
Target: Polka API
Potential Vulnerability

Your Polka API might be exposed to JWT Vulnerabilities (Weak Signing, None Algo)

74% of Polka apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.

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