JWT Token Utility
Decode · Modify · Re-Encode
FOR AUTHORIZED TESTING ONLY — Use only on systems you own or have explicit permission to test.
Paste a JWT token and click Decode
Click Re-Encode to generate modified token
JWT Security Considerations
Transmission Risk
Always use HTTPS to prevent token interception
Storage Risk
localStorage → XSS, cookies → CSRF. Use HttpOnly + Secure cookies
Validation Risk
Reject "none" algorithm. Verify signatures strictly
Replay Attack
Use short expiry (exp claim) + refresh tokens
Tampering Risk
Header/payload changes require valid signature
Leakage Risk
Never log or expose tokens in URLs or client-side storage
Algorithm Confusion
Enforce expected algorithm (RS256 ≠ HS256)
Claims Manipulation
Always validate claims server-side, never trust client