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testing-for-json-web-token-vulnerabilities

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Test JWT implementations for critical vulnerabilities including algorithm confusion, none algorithm bypass, kid parameter injection, and weak secret exploitation to achieve authentication bypass and privilege escalation.

internal/tools/skills/data/web-application-security/testing-for-json-web-token-vulnerabilities/SKILL.md

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# Testing for JSON Web Token Vulnerabilities

## When to Use
- When testing applications using JWT for authentication and session management
- During API security assessments where JWTs are used for authorization
- When evaluating OAuth 2.0 or OpenID Connect implementations using JWT
- During penetration testing of single sign-on (SSO) systems
- When auditing JWT library configurations for known vulnerabilities

### How to CONFIRM a Hit (avoid false negatives)

- **Positive signal**: a token YOU forged (alg=none, RS256→HS256 confusion, cracked HMAC secret, or kid/jku/x5u abuse) is **ACCEPTED by a protected endpoint** — returns 200 plus the privileged data/action for the impersonated user. Merely decoding or modifying the token proves nothing.
- Confirm by hitting a real authz-gated route (e.g. `/api/admin/*`) with the forged token and verifying you receive the restricted resource, not a generic page or 401/403.
- A 200 to a public route is NOT a hit; a single 401/403 is NOT a clean negative until you have varied the attack.