| name | web-auth-jwt |
| description | Attack JWT/session authentication. Load when auth uses a JWT (three base64url parts, header.payload.signature), Authorization: Bearer, or you see alg/kid/jku fields. Signals: eyJ... tokens, "alg":"none"/"HS256"/"RS256", kid header, JWKS endpoints, role/admin claims.
|
| domain | web |
| type | technique |
| stability | learning |
| modes | ["pentest","bugbounty"] |
| severity | high |
| owasp | ["A07:2021-Auth-Failures"] |
| owasp_api | ["API2:2023-Broken-Authentication"] |
| cwe | ["CWE-347","CWE-287"] |
| tools | ["jwt_tool","hashcat","burp"] |
| schema_version | 1 |
JWT / token authentication attacks
When it applies
The app authenticates with a JSON Web Token and trusts claims inside it (user id, role,
isAdmin). The win is forging a valid-looking token the server accepts.
Why it works
JWTs are only as safe as their signature verification. Implementations routinely skip it,
confuse algorithms, or trust attacker-controlled key hints (kid, jku, x5u). If you can
make the server accept a signature you produced, you own every claim.
Method
- Decode & read claims:
jwt_tool <token> — look for role, admin, sub, weak exp.
- alg=none: strip the signature and set header
{"alg":"none"}; some libs accept it.
- Algorithm confusion RS256→HS256: if the server verifies RS256 with the public key,
re-sign HS256 using that public key as the HMAC secret — server verifies with the same key.
jwt_tool -X k -pk public.pem.
- Weak HMAC secret: crack HS256 offline —
hashcat -m 16500 token.txt rockyou.txt; if it
falls, mint any token.
- kid / jku / x5u abuse:
kid path traversal or SQLi to control the key file; jku/x5u
pointing at your JWKS so the server fetches your public key and you sign with your private key.
Gotchas
- Editing claims without re-signing fails unless verification is broken — confirm the vuln class first.
exp/nbf may be enforced even when signature isn't — keep timestamps valid.- Public key sourcing: grab it from
/.well-known/jwks.json, a cert, or TLS if not published.
Verify success
Server accepts a token you forged with elevated claims (e.g. admin panel loads, or an
authenticated endpoint returns another user's data with your minted sub).
References
PortSwigger JWT labs; jwt_tool wiki; Auth0 "Critical vulnerabilities in JWT libraries".
