Fix JWT Vulnerabilities (Weak Signing, None Algo) in Gatsby
JWT implementation in Gatsby Serverless Functions is a common target for authentication bypass. Two critical failures are frequently observed: accepting the 'none' algorithm and using weak, hardcoded secrets. If your verification logic doesn't explicitly enforce the signing algorithm, an attacker can modify the payload and bypass authentication entirely by zeroing out the signature and setting the 'alg' header to 'none'.
The Vulnerable Pattern
const jwt = require('jsonwebtoken');
export default function handler(req, res) { const token = req.headers.authorization?.split(’ ’)[1]; // VULNERABLE: No algorithm enforcement and weak, hardcoded secret. // An attacker can change the header to {“alg”:“none”} and bypass verification. try { const decoded = jwt.verify(token, ‘secret123’); res.status(200).json({ user: decoded.sub }); } catch (err) { res.status(401).send(‘Unauthorized’); } }
The Secure Implementation
The 'none' algorithm vulnerability allows an attacker to forge tokens by stripping the signature and updating the header. In Gatsby's Node.js environment, the 'jsonwebtoken' library requires explicit algorithm whitelisting to mitigate this. Additionally, weak secrets (like 'secret123') are susceptible to offline brute-force attacks using tools like Hashcat. By enforcing 'algorithms: ["HS256"]' and using a 256-bit secret stored in a secure environment variable, you eliminate the possibility of algorithm-switching and signature-stripping exploits.
const jwt = require('jsonwebtoken');export default function handler(req, res) { const authHeader = req.headers.authorization; if (!authHeader) return res.status(401).send(‘Missing Authorization Header’);
const token = authHeader.split(’ ’)[1];
try { // SECURE: Enforce HS256 algorithm and use a high-entropy secret from environment variables. // This prevents ‘alg: none’ bypass and algorithm switching attacks (e.g., RS256 to HS256). const decoded = jwt.verify(token, process.env.JWT_SIGNING_KEY, { algorithms: [‘HS256’], issuer: ‘gatsby-app-prod’, audience: ‘api-access’ });
res.status(200).json({ user: decoded.sub });
} catch (err) { res.status(401).json({ error: ‘Invalid or Expired Token’ }); } }
Your Gatsby API
might be exposed to JWT Vulnerabilities (Weak Signing, None Algo)
74% of Gatsby apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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