SKILL: OAuth Security Testing

Metadata

• Skill Name: oauth-attacks
• Folder: offensive-oauth
• Source: https://github.com/SnailSploit/offensive-checklist/blob/main/oauth.md

Description

OAuth 2.0 attack checklist: authorization code interception, redirect_uri bypass, CSRF on OAuth flow, state parameter abuse, open redirector chaining, token leakage via Referer, PKCE bypass, and scope escalation. Use when testing OAuth implementations in web apps or bug bounty.

Trigger Phrases

Use this skill when the conversation involves any of: OAuth, OAuth 2.0, authorization code, redirect_uri bypass, OAuth CSRF, state parameter, PKCE bypass, scope escalation, token leakage, open redirector, OAuth attack

Instructions for Claude

When this skill is active: 1. Load and apply the full methodology below as your operational checklist 2. Follow steps in order unless the user specifies otherwise 3. For each technique, consider applicability to the current target/context 4. Track which checklist items have been completed 5. Suggest next steps based on findings

Full Methodology

OAuth Security Testing

Shortcut

• Check for improper redirect validation (open redirects)
• Test state parameter manipulation/absence
• Manipulate OAuth flows to bypass authentication
• Try URL path traversal in redirect_uri
• Hunt for client secret leakage in source code/repos
• Look for improper scope validation

Mechanisms

• OAuth 2.0 authorizes limited access to resources via tokens; pair with OIDC for identity.
• Core Flows:
• Authorization Code (with PKCE for public clients)
• Client Credentials (service-to-service)
• Avoid Implicit and ROPC where possible
• Key Components:
• Resource Owner (user)
• Client (third-party app)
• Authorization Server (issues tokens)
• Resource Server (hosts protected resources)
• Tokens (access and refresh)
• Hardening Extensions:
• PAR (Pushed Authorization Requests), JAR (Request Objects), JARM (JWT-secured responses)
• Sender‑constrained tokens (DPoP, mTLS)
• private_key_jwt or mTLS client authentication for confidential clients

OAuth/OIDC Considerations

• PKCE everywhere: Even with confidential clients/native apps; code_verifier must be required and validated.
• Nonce/state binding: For OIDC, ensure nonce is present and matched; state should be unguessable and tied to session.
• redirect_uri exact match: Enforce exact string match against pre-registered allowlist; no wildcards/path traversal.
• aud/azp/iss enforcement: Validate tokens strictly, including clock skew and JWKS kid rotation behavior.
• Front-channel logout/login CSRF: Validate logout CSRF; defend forced login to attacker accounts.
• ID Token vs Access Token: APIs must not accept ID tokens; check token_type and audience.
• Device Code & CIBA: Validate polling rate limits, code expiry, and binding of device/user codes.
• Refresh Token Rotation: Enforce reuse detection and global invalidation chains.
• PAR/JAR/JARM: Use to pin exact redirect_uri and inputs and to protect front-channel parameters.

OAuth 2.1 Updates

• Implicit Flow Deprecated: Authorization servers should not support response_type=token
• Password Grant Deprecated: ROPC (Resource Owner Password Credentials) considered insecure
• PKCE Mandatory: Required for all OAuth clients including confidential clients
• Exact Redirect URI Matching: No more substring or prefix matching allowed
• Refresh Token Sender Constraint: Refresh tokens should be sender-constrained via DPoP or mTLS

Financial-grade API (FAPI) Security

FAPI 1.0 Advanced Profile

• Signed Request Objects (JAR): Authorization requests as signed JWTs
• Hybrid Flow: Uses response_type=code id_token for additional security
• MTLS Client Authentication: Certificate-bound tokens
• JARM: JWT-secured authorization response mode
• Request Object Encryption: Sensitive parameters encrypted

FAPI 2.0 Security Profile

• Pushed Authorization Requests (PAR): POST request parameters to dedicated endpoint
• DPoP (Demonstrating Proof-of-Possession): Token bound to client's key pair
• Client Authentication: private_key_jwt or MTLS required
• Grant Management: Rich authorization requests and grant management API

```mermaid graph TD User[Resource Owner] -->|Initiates flow| Client Client -->|Authorization Request| AuthServer[Authorization Server] AuthServer -->|Authentication| User User -->|Approves access| AuthServer AuthServer -->|Authorization Code| Client Client -->|Code + Client Secret| AuthServer AuthServer -->|Access Token| Client Client -->|Access Token| ResourceServer[Resource Server] ResourceServer -->|Protected Resource| Client

style User fill:#b7b,stroke:#333,color:#333
style Client fill:#aae,stroke:#333,color:#333
style AuthServer fill:#9f9,stroke:#333,color:#3