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

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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

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:#333
    style ResourceServer fill:#e9a,stroke:#333,color:#333

Hunt

• Intercept OAuth flows with proxy (Burp/ZAP)
• Manipulate redirect_uri parameters
• Remove/tamper state parameter
• Test PKCE implementations
• Inspect token handling in browsers
• Check for client secret leakage
• Analyze scope handling logic
• Test account linking/unlinking
• Review token validation procedures
• Examine refresh token security

Native/Mobile

• Verify App Links/Universal Links to prevent hijacking callbacks.
• Ensure OAuth proxy components in mobile apps validate issuer and JWKS; do not ship client secrets in binaries.

SPA/Browser

• Use Authorization Code + PKCE; avoid Implicit/Hybrid unless justified.
• Store tokens in memory; if cookies are used, set __Host- prefix with HttpOnly; Secure; SameSite.

Authorization Code Flow

• Initial authorization request has response_type=code
• Request format: /authorization?client_id=12345&redirect_uri=https://client-app.com/callback&response_type=code&scope=openid%20profile&state=ae13d489bd00e3c24
• Callback contains authorization code: /callback?code=a1b2c3d4e5f6g7h8&state=ae13d489bd00e3c24
• More secure, backend exchanges code for tokens

sequenceDiagram
    participant User
    participant Client
    participant AuthServer as Authorization Server
    participant API as Resource Server

    User->>Client: 1. Click "Login with Service"
    Client->>AuthServer: 2. Authorization Request (response_type=code)
    AuthServer->>User: 3. Login & Consent
    User->>AuthServer: 4. Approves Access
    AuthServer->>Client: 5. Redirect with Authorization Code
    Client->>AuthServer: 6. Token Request (code + client_secret)
    AuthServer->>Client: 7. Access & Refresh Tokens
    Client->>API: 8. API Request + Access Token
    API->>Client: 9. Protected Resource

Implicit Flow

• Initial authorization request has response_type=token
• Request format: /authorization?client_id=12345&redirect_uri=https://client-app.com/callback&response_type=token&scope=openid%20profile&state=ae13d489bd00e3c24
• Access token returned directly in URL fragment: /callback#access_token=z0y9x8w7v6u5&token_type=Bearer&expires_in=5000&scope=openid%20profile&state=ae13d489bd00e3c24
• Higher vulnerability potential due to frontend token handling

sequenceDiagram
    participant User
    participant Client as Client (Browser)
    participant AuthServer as Authorization Server
    participant API as Resource Server

    User->>Client: 1. Click "Login with Service"
    Client->>AuthServer: 2. Authorization Request (response_type=token)
    AuthServer->>User: 3. Login & Consent
    User->>AuthServer: 4. Approves Access
    AuthServer->>Client: 5. Redirect with Access Token in Fragment
    Note over Client: Token stored in browser
    Client->>API: 6. API Request + Access Token
    API->>Client: 7. Protected Resource

Vulnerabilities

• Improper redirect_uri validation
  • Open redirects
  • Subdomain/path validation bypass
• CSRF attacks (missing/improper state parameter)
• Token leakage (URL fragments in referrer headers)
• Scope elevation (improper authorization)
• Account takeover via improper linking/unlinking
• JWT vulnerabilities (weak signatures, lack of validation)
• Client secret exposure in source/git repositories
• Authorization bypass in misconfigured implementations
• Session fixation attacks
• Access token theft via XSS/Man-in-the-Middle

Authorization Code Injection / Code Substitution

• Attacker injects victim authorization code into attacker session to bind victim account. Mitigate with state-nonce binding and PKCE.

Method 1: Auth Bypass in OAuth Implicit Flow

• Locate POST request containing user info (email, username) and access token
• In implicit flow, servers often don't properly validate access tokens
• Try changing user parameters (email, username) while keeping the token
• Potentially impersonate other users if server trusts client-provided identifiers

Method 2: Forced Profile Linking

• Target OAuth profile linking functionality
• Check for missing state parameter in auth requests
• Create CSRF attack by copying auth URL before code/token use
• Deliver as direct link or embedded iframe to victim
• Can link attacker's social media to victim's account

Method 3: Account Hijacking via redirect_uri

• Identify authorization request with redirect_uri parameter
• Test redirect_uri manipulation (external domains or open redirects)
• Modify redirect_uri to attacker-controlled endpoint (webhook)
• Deliver modified auth URL to victim to capture their authorization code
• Use stolen code to complete OAuth flow and access victim's account

Methodologies

• Tools:
  • Burp Suite (OAuth Scanner extension)
  • OWASP ZAP
  • OAuth 2.0 Threat Model Toolkit
  • Postman for API testing
  • JWT_Tool for token analysis
  • OAuthSecurity Cheatsheet Scanner
• Techniques:
  • Flow manipulation
  • Parameter tampering
  • Token analysis
  • Replay attacks
  • Social engineering (phishing for tokens)
  • DPoP proof validation testing
  • MTLS certificate validation testing
  • PAR endpoint exploitation
  • Token exchange flow testing

Chaining and Escalation

OAuth → Full Account Takeover

1. Open Redirect → Authorization Code Theft:

   • Discover open redirect on trusted domain
   • Craft OAuth flow with redirect_uri pointing to open redirect
   • Victim clicks malicious link, completes OAuth flow
   • Authorization code redirected through open redirect to attacker
   • Attacker exchanges code for access token

2. CSRF → Account Linking Attack:

   • Initiate OAuth flow to link social account
   • Capture authorization callback URL before code is used
   • Deliver URL to victim via CSRF
   • Victim's account linked to attacker's social account
   • Attacker logs in with social account to access victim's account

3. XSS → Token Theft:

   • Find XSS vulnerability on application
   • Inject script to steal access tokens from localStorage
   • Use stolen tokens to access victim's API resources
   • If refresh tokens stolen, maintain persistent access

OAuth → Lateral Movement

1. Token Exchange → Service Impersonation:

   • Obtain low-privilege access token
   • Use RFC 8693 token exchange to request token for different service
   • Weak validation allows unauthorized service access
   • Move laterally across microservices

2. Scope Elevation → Privilege Escalation:

   • Obtain token with limited scope
   • Manipulate refresh token exchange to request broader scopes
   • Weak scope validation grants elevated permissions
   • Access privileged API endpoints

3. IdP Confusion → Cross-Tenant Access:

   • Multi-tenant application with multiple IdPs
   • Obtain authorization code from Tenant A's IdP
   • Exchange code at Tenant B's token endpoint
   • Weak issuer validation grants cross-tenant access

OAuth → Backend Exploitation

1. JWT Algorithm Confusion → Signature Bypass:

   • Obtain valid JWT access token
   • Change algorithm from RS256 to HS256
   • Sign token with public key (treating it as HMAC secret)
   • Backend fails to validate algorithm properly
   • Forge arbitrary tokens for privilege escalation

2. SSRF via redirect_uri → Internal Service Access:

   • OAuth provider allows internal redirect_uri
   • Set redirect_uri to internal service (http://169.254.169.254)
   • Authorization response sent to internal service
   • Use to access cloud metadata or internal APIs

3. Token Replay → Session Hijacking:

   • Capture access token via network sniffing or logs
   • Token not properly bound to client (no DPoP/MTLS)
   • Replay token from attacker's system
   • Hijack victim's session and access resources

Remediation Recommendations

OAuth 2.1 / Modern Implementation

• Implement OAuth 2.1: Adopt latest security recommendations

  • Deprecate Implicit and Password grants
  • Require PKCE for all clients (public and confidential)
  • Enforce exact redirect_uri matching
  • Implement refresh token rotation with reuse detection

• Enforce state parameter: Always required, cryptographically random, single-use

• Validate token claims strictly:

  • aud (audience): Must match resource server
  • iss (issuer): Verify against known issuers
  • exp (expiration): Enforce with clock skew tolerance (max 60s)
  • nbf (not before): Validate if present

• Secure token storage:

  • Never use localStorage (XSS vulnerable)
  • Use httpOnly cookies with __Host- prefix or memory-only storage
  • Set proper cookie flags: HttpOnly; Secure; SameSite=Strict

Advanced Security Features

• Implement PAR (Pushed Authorization Requests): POST parameters to /par endpoint
• Use DPoP (Demonstrating Proof-of-Possession): Bind access tokens to client's public key
• Implement MTLS for confidential clients: Certificate-bound access tokens
• Use JAR (JWT-secured Authorization Request): Sign authorization request parameters
• Consider JARM (JWT-secured Authorization Response): Signed authorization responses

Token Management

• Short-lived access tokens: 5-15 minutes maximum
• Refresh token rotation: Issue new refresh token on each use
• Refresh token reuse detection: Revoke entire token family on reuse
• Token binding: Use DPoP or MTLS to bind tokens to clients

Standards and Compliance

• Follow OAuth 2.1 (draft) guidance
• Implement FAPI if dealing with financial data
• Follow RFC 6819 OAuth threat model
• Adopt RFC 8252 for native apps
• Consider RFC 8693 for secure token exchange
• Implement RFC 9449 for DPoP

Regular Security Practices

• Rotate signing keys regularly (every 6-12 months)
• Implement JWKS with short TTL (< 1 hour)
• Pin trusted issuers in client configuration
• Conduct regular OAuth security audits
• Keep libraries and dependencies updated