JWT Security Mistakes That Put Your API at Risk
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JWT is easy to get working and surprisingly easy to get wrong in ways that don't show up until someone actually tries to exploit them. These four mistakes account for most of the real-world JWT vulnerabilities I see, across every language and framework — the framework isn't the risk, the implementation choices are.
Mistake 1: Algorithm confusion
A JWT's header declares which algorithm was used to sign it. If your server trusts that header blindly instead of enforcing an expected algorithm, an attacker can manipulate it — in the worst documented cases, tricking a server expecting an asymmetric algorithm (RS256) into verifying a forged token with its own public key treated as an HMAC secret.
// ✓ Safe — algorithm explicitly restricted
jwt.verify(token, secretKey, { algorithms: ['HS256'] });
// ✗ Risky — no algorithm restriction, trusts the token's own header
jwt.verify(token, secretKey);
Mistake 2: A weak or shared signing secret
If your secret is short, guessable, or reused across environments, an attacker who obtains or brute-forces it can forge any token they want. Generate a genuinely random, long secret, and use different secrets for staging and production:
Store the result as an environment variable, never as a literal string in source code — the same rule that applies to any credential.
Mistake 3: Sensitive data in the payload
A JWT payload is base64-encoded, not encrypted. Anyone who has the token can decode it instantly:
{ "userId": 42, "ssn": "123-45-6789", "creditCard": "4111..."}
// ✓ Non-sensitive identifiers only
{ "userId": 42, "role": "editor", "exp": 1735689600 }
Mistake 4: No expiry, or an expiry that's too long
A token with no exp claim is valid forever — a stolen token becomes permanent access rather than a limited window. Short-lived access tokens (15–60 minutes) combined with a longer-lived refresh token is the pattern to actually use, the same one covered in an earlier post on JWT authentication in Laravel — the principle applies identically regardless of which framework issues the token.
A quick self-check
Want a real security review of your authentication system, whatever framework it's built on? This is part of what I cover in DevOps services engagements.