Troubleshooting JWT Token Issues
JSON Web Tokens are widely used for authentication but can be frustrating to debug. This guide covers common JWT problems including expiration errors, signature mismatches, and payload decoding issues.
Key Takeaways
- A JWT consists of three Base64URL-encoded parts separated by dots: header, payload, and signature.
- Symptom: `TokenExpiredError` or `jwt expired` error.
- This is also a known security vulnerability โ never trust the `alg` claim from untrusted tokens.
- Never let the JWT itself dictate which algorithm to use.
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JWT Structure
A JWT consists of three Base64URL-encoded parts separated by dots: header, payload, and signature. Understanding this structure is key to debugging.
Common Issues
Token Expired
Symptom: TokenExpiredError or jwt expired error.
Cause: The exp (expiration) claim in the payload is in the past. This can happen due to clock skew between servers, or because the token's lifetime is too short.
Fix: Check the exp claim by decoding the payload (without verification). Compare with the current time. Add a clock tolerance (leeway) of 30-60 seconds to account for clock differences.
Invalid Signature
Symptom: JsonWebTokenError: invalid signature.
Cause: The signing key used to verify doesn't match the key used to create the token. This happens when keys are rotated, when using the wrong key for the algorithm, or when the token was tampered with.
Fix: Ensure you're using the correct signing key. For RS256, verify you're using the public key (not private) for verification. Check the kid (key ID) claim in the header.
Algorithm Confusion
Symptom: Signature verification fails despite correct keys.
Cause: The alg header says RS256 but verification uses HS256, or vice versa. This is also a known security vulnerability โ never trust the alg claim from untrusted tokens.
Fix: Always specify the expected algorithm explicitly in your verification code. Never let the JWT itself dictate which algorithm to use.
Debugging Steps
- Decode the header and payload (Base64URL) without verification.
- Check
exp,iat, andnbftimestamps. - Verify the
iss(issuer) andaud(audience) claims. - Confirm the algorithm matches your verification setup.
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