Hunt Jwt Crypto
Hunt JWT cryptographic failures, alg:none signature-stripping and RS256→HS256...
Install
npx skills add elementalsouls/Claude-BugHunter --skill hunt-jwt-crypto -g -a claude-code -y- Repository
- elementalsouls/Claude-BugHunter5.4k
- Category
- Engineering
- Updated
- 3d ago
- License
- MIT
- Context cost
- Mediumabout 2,090 tokens
- Runs scripts
- No, instructions only
What it does
Hunt JWT cryptographic failures, alg:none signature-stripping and RS256→HS256 key-confusion that let an attacker forge a token for any identity (e.g. an admin) without knowing a secret. Use when the app authenticates with a JSON Web Token (an `eyJ...` Bearer token in the Authorization header, a cookie, or a login response). This skill OWNS JWT signature/crypto forgery (alg:none, key confusion, kid/jku header injection); hunt-ato covers JWT as one ATO path, hunt-auth-bypass covers SSO/SAML token trust, hunt-api-misconfig covers non-crypto JWT handling. Critical when a forged token grants access to another user's data or an admin-only endpoint.
HUNT-JWT-CRYPTO, Forgeable JSON Web Tokens (A04 Cryptographic Failures)
What actually pays
A JWT is header.payload.signature, each base64url. The signature is the only
thing stopping you from editing the payload (your identity/role) and replaying
it. It pays High/Critical when the verifier can be tricked into accepting a
token you forged, so you become another user or an admin without their secret.
Two classic, generic verifier flaws:
alg:none, the verifier trusts the token's ownalgheader. Setalg:"none", drop the signature, edit the payload (e.g.role:"admin", another user'sid/email). A broken verifier skips signature checking.- RS256 → HS256 key confusion, the token is signed RS256 (asymmetric). The RSA public key is, by definition, public. If the verifier lets you choose HS256, it will use that public key as the HMAC secret, which you also know. Sign an edited payload with HS256 using the public key and it validates.
Recon, is this app JWT-based?
Login/token responses containing "token":"eyJ..." or Set-Cookie: token=eyJ...
Authorization: Bearer eyJ... on authenticated requests
A JWKS / public-key endpoint: /.well-known/jwks.json, /jwks, public-key in the JS bundle
Decode the header (base64url the first segment). "alg":"RS256" → try key
confusion. Any alg → always try alg:none first; it's free.
Forging the token (never hand-encode base64, use a JWT tool)
Use a purpose-built tool so encoding/signing is correct: jwt_tool
(jwt_tool <token> -T to tamper interactively, -X a for alg:none, -X k -pk public.pem for key confusion), Burp's JWT Editor extension, or a few lines
of PyJWT. Each forge below is the concept plus the claim to edit.
