# Offensive Jwt

JWT attack methodology for penetration testers.

- Repository: https://github.com/SnailSploit/Claude-Red (10186 stars)
- Category: Testing
- License: MIT
- Last updated: 2026-10-12
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add SnailSploit/Claude-Red --skill offensive-jwt -g -a claude-code -y`
Codex: `npx skills add SnailSploit/Claude-Red --skill offensive-jwt -g -a codex -y`
Cursor: `npx skills add SnailSploit/Claude-Red --skill offensive-jwt -g -a cursor -y`
Gemini CLI: `npx skills add SnailSploit/Claude-Red --skill offensive-jwt -g -a gemini-cli -y`

## Description

JWT attack methodology for penetration testers. Covers algorithm confusion (alg:none, RS256→HS256), weak HMAC secret brute force, kid parameter injection (SQLi, path traversal), jku/x5u/jwk header injection, JWKS cache poisoning, JWS/JWE confusion, timing attacks, and mobile JWT storage extraction. Use when testing JWT-based authentication, hunting auth bypass via token manipulation, or evaluating JWT implementation security in web or mobile apps.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

## 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](https://github.com/ticarpi/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

The rest: https://github.com/SnailSploit/Claude-Red/blob/HEAD/Skills/auth/offensive-jwt/SKILL.md