agents: Skill for Claude Code

JWT Vulnerability Detection

You are performing a focused security assessment to find insecure JSON Web Token (JWT) implementations. This skill uses a two-phase approach with subagents: recon (map the full JWT lifecycle — issuance, verification, and configuration) then analysis (identify every exploitable weakness in those verification sites).

Prerequisites: sast/architecture.md must exist. Run the analysis skill first if it doesn't.

---

What is an Insecure JWT Implementation

JWTs consist of three Base64URL-encoded parts: header.payload.signature. The header declares the signing algorithm (alg), the payload carries claims (e.g., sub, role, exp), and the signature is a cryptographic proof of integrity. Vulnerabilities arise when the server trusts the token's own claims about how it was signed, fails to verify the signature at all, uses a guessable secret, or trusts attacker-controlled key material embedded in the token itself.

The core pattern: the server does not fully verify the JWT's authenticity and integrity before trusting its claims.

What JWT Vulnerabilities ARE

1. Algorithm confusion — alg: none The server accepts a JWT whose header declares "alg": "none", bypassing signature verification entirely. An attacker crafts an arbitrary payload, sets alg to none, and omits the signature. If the library processes it, the forged token is accepted.

2. Algorithm confusion — RS256 → HS256 A server configured for RS256 (asymmetric: sign with private key, verify with public key) can be tricked into HS256 mode if the library allows the algorithm to be specified by the token. Since the public key is often retrievable, the attacker signs a forged token with HS256 using the server's public key as the HMAC secret. The server verifies the HMAC using the same public key and accepts the token.

3. Missing or disabled signature verification The server decodes the JWT payload without actually verifying the signature. Common patterns:

• Python (PyJWT): jwt.decode(token, options={"verify_signature": False})
• Node.js (jsonwebtoken): jwt.decode(token) instead of jwt.verify(token, secret)
• Manual base64 decode of the payload with no signature check
• algorithms=["none"] accepted in the decode call