offensive-jwt

Overview

Comprehensive JWT attack checklist for offensive security engagements. Follow steps in order; apply each technique to the current target context and track which items have been completed.

Quick Reference: Misconfigurations to Check

• Algorithm set to none — signature verification bypassed entirely
• Algorithm switching between RSA and HMAC (confusion attack)
• Weak or guessable HMAC secret (brute-forceable)
• kid, jku, jwk, x5u header parameters accepted without validation
• Expired or tampered tokens accepted by server
• Sensitive data stored unencrypted in payload

Useful tool: JWT Tool

Mechanisms

JWTs (RFC 7519) consist of three Base64URL-encoded parts: header.payload.signature.

Signing algorithms:

Algorithm	Type	Notes
HS256/384/512	Symmetric HMAC	Shared secret; confusion target
RS256/384/512	Asymmetric RSA	Public key can be misused as HMAC secret
ES256/384/512	Asymmetric ECDSA
PS256/384/512	RSASSA-PSS
EdDSA (Ed25519/Ed448)	Asymmetric
none	Unsigned	Critically insecure

Additional pitfalls:

• JWS/JWE confusion: server accepts encrypted token (JWE) where signed (JWS) is expected, or fails open on unexpected typ/cty
• JWKS retrieval: SSRF via jku/x5u, insecure TLS, poisoned key caching, kid collisions
• Token binding (DPoP, mTLS): incorrectly implemented allows replay from other clients

Hunt: Identifying JWT Usage

1. Check Authorization: Bearer <token> headers in all requests
2. Look for cookies containing JWT structures (eyJ...)
3. Examine browser local/session storage
4. Decode the token at jwt.io or via BurpSuite JWT extension — inspect claims and header parameters
5. Note any kid, jku, jwk, x5u fields in the header — these are attack surfaces

Vulnerability Map

JWT Vulnerabilities
├── Algorithm Bypass
│   ├── alg:none attack
│   └── RS256→HS256 confusion (public key as HMAC secret)
├── Weak Secret Key → Brute force
├── kid Parameter Injection
│   ├── SQL injection via kid
│   └── Path traversal via kid
├── Header Injection
│   ├── jwk (inline fake key)
│   ├── jku/x5u (remote attacker-controlled JWKS)
│   └── JWKS cache poisoning
└── Missing / Broken Validation
    ├── No signature check
    ├── Expired tokens accepted
    └── iss/aud/exp not validated