How to Debug JWT Tokens Like a Pro

JSON Web Tokens (JWT) are everywhere in modern web development. Whether you're building authentication systems, API integrations, or microservices, understanding how to properly debug JWT tokens is crucial.

What is a JWT Token?

A JWT token consists of three parts separated by dots (.):

header.payload.signature

• Header: Contains the token type and hashing algorithm
• Payload: Contains the claims (user data, expiration, etc.)
• Signature: Verifies the token hasn't been tampered with

Common JWT Issues

1. Token Expiration

The most common JWT error. Tokens include an exp claim that specifies when they expire.

1{
2  "exp": 1705132800,
3  "iat": 1705129200,
4  "sub": "user123"
5}

Solution: Use our JWT Debugger to check the expiration timestamp and compare it with the current time.

2. Invalid Signature

If the token signature doesn't match, it means either:

• The token was modified after signing
• You're using the wrong secret key
• The token was signed with a different algorithm

Debug Steps:

1. Decode the header to check the algorithm (alg claim)
2. Verify you're using the correct secret key
3. Ensure the signing method matches

3. Malformed Token Structure

A valid JWT must have exactly 3 parts (header, payload, signature). Common issues:

• Missing dots (.)
• Extra whitespace
• URL encoding issues

Quick Fix: Copy-paste your token into our JWT Debugger to instantly see if it's properly formatted.

Using Developer Toolkit's JWT Debugger

Our JWT Debugger makes troubleshooting easy:

Step 1: Paste Your Token

Simply copy the JWT token from your application or API response and paste it into the input field.

Step 2: View Decoded Data

The tool automatically decodes and displays:

• Header information (algorithm, token type)
• Payload claims (user ID, roles, expiration)
• Signature verification status

Step 3: Check Expiration

The debugger highlights if your token is expired and shows both:

• Human-readable expiration time
• Unix timestamp

Step 4: Validate Structure

Get instant feedback on:

• Token format validity
• Required claims presence
• Timestamp consistency

Best Practices

1. Always Use HTTPS: Never send JWT tokens over unencrypted connections
2. Set Short Expiration Times: Reduce risk if tokens are compromised
3. Implement Token Refresh: Use refresh tokens for long-lived sessions
4. Validate on Every Request: Always verify tokens on the backend
5. Use Strong Secrets: Never use weak or default secret keys

Advanced Debugging Tips

Check Token Size

JWT tokens should be compact. If your token is unusually large (>1KB), consider:

• Reducing payload claims
• Using token references instead of embedding data
• Implementing token compression

Verify Claims

Essential claims to check:

• iss (issuer): Who created the token
• sub (subject): Who the token is about
• aud (audience): Who the token is intended for
• exp (expiration): When the token expires
• nbf (not before): When the token becomes valid
• iat (issued at): When the token was created

Debug Token Flow

1. Creation: Verify token generation is correct
2. Transmission: Check how token is sent (header, cookie, body)
3. Storage: Ensure secure storage (HttpOnly cookies, secure storage)
4. Validation: Confirm proper signature verification
5. Expiration: Handle token refresh gracefully

Common Error Messages

"Token expired"

Cause: The exp claim is in the past Fix: Request a new token or implement token refresh

"Invalid signature"

Cause: Signature verification failed Fix: Check your secret key and algorithm

"Token malformed"

Cause: Invalid JWT structure Fix: Verify the token has 3 parts separated by dots

"Algorithm not allowed"

Cause: Security restriction on algorithm Fix: Use HS256, RS256, or ES256

Security Considerations

⚠️ Never decode JWT tokens on untrusted websites. Always use:

• Your own tools
• Trusted developer tools like Developer Toolkit
• Local/offline tools when handling sensitive tokens

Our JWT Debugger processes everything client-side - your tokens never leave your browser.

Try It Now

Ready to debug your JWT tokens? Head over to our JWT Debugger and start troubleshooting instantly.

Related Tools

• Base64 Encoder/Decoder - Decode JWT parts manually
• JSON Formatter - Format JWT payload
• CSRF Token Tester - Test security tokens

Have questions or suggestions? We're always improving our tools based on developer feedback!