testing-jwt-token-security

Decode the token to inspect its header and payload, then test for algorithm none acceptance, RS256-to-HS256 confusion, and weak HMAC secrets using jwt_tool or the included Python agent. Also verify claim tampering, expired token acceptance, and revocation after logout.

jwt_tool provides automated attack modules, Burp Suite's JWT Editor enables real-time token manipulation, and Hashcat (mode 16500) or John the Ripper crack HMAC secrets. PyJWT supports scripted token forging and manipulation.

If a server accepts both RS256 and HS256, an attacker retrieves the public RSA key (often from /.well-known/jwks.json) and uses it as the HMAC secret to sign a forged HS256 token. The server verifies it with the same public key, accepting the forgery.

Yes, HS256 tokens signed with dictionary words or short secrets can be cracked offline using Hashcat or John the Ripper with wordlists like rockyou.txt. Once recovered, the secret allows forging arbitrary tokens with any claims.

JWTs are stateless, so servers cannot invalidate them without a server-side revocation list or blocklist. If the application only checks the signature and expiration, captured tokens remain usable until they expire, which is a reportable finding.