Anthropic-Cybersecurity-Skills/skills/testing-mobile-api-authentication/SKILL.md

---
name: testing-mobile-api-authentication
description: 'Tests authentication and authorization mechanisms in mobile application APIs to identify broken authentication,
  insecure token management, session fixation, privilege escalation, and IDOR vulnerabilities. Use when performing API security
  assessments against mobile app backends, testing JWT implementations, evaluating OAuth flows, or assessing session management.
  Activates for requests involving mobile API auth testing, token security assessment, OAuth mobile flow testing, or API authorization
  bypass.

  '
domain: cybersecurity
subdomain: mobile-security
author: mahipal
tags:
- mobile-security
- android
- ios
- api-security
- authentication
- penetration-testing
version: 1.0.0
license: Apache-2.0
nist_csf:
- PR.PS-01
- PR.AA-05
- ID.RA-01
- DE.CM-09
---
# Testing Mobile API Authentication

## When to Use

Use this skill when:
- Assessing mobile app backend API authentication during penetration tests
- Testing JWT token implementation for common vulnerabilities (none algorithm, weak signing)
- Evaluating OAuth 2.0 / OIDC flows in mobile applications for redirect, PKCE, and scope issues
- Testing for broken object-level authorization (BOLA/IDOR) in API endpoints

**Do not use** this skill against production APIs without explicit authorization and rate-limiting awareness.

## Prerequisites

- Burp Suite or mitmproxy configured as mobile device proxy
- SSL pinning bypassed on target application (if implemented)
- Valid test account credentials for the target application
- Postman or curl for API request crafting
- jwt.io or PyJWT for JWT analysis and manipulation

## Workflow

### Step 1: Map Authentication Endpoints

Intercept mobile app traffic to identify authentication-related endpoints:

```
POST /api/v1/auth/login          - Initial authentication
POST /api/v1/auth/register       - Account registration
POST /api/v1/auth/refresh        - Token refresh
POST /api/v1/auth/logout         - Session termination
POST /api/v1/auth/forgot-password - Password reset
POST /api/v1/auth/verify-otp     - OTP verification
GET  /api/v1/auth/me             - Authenticated user profile
```

### Step 2: Analyze Token Format and Security

**JWT Analysis:**
```bash
# Decode JWT without verification
echo "eyJhbGciOiJIUzI1NiIs..." | cut -d. -f2 | base64 -d 2>/dev/null

# Check for common JWT vulnerabilities:
# 1. None algorithm attack
# Change header to: {"alg":"none","typ":"JWT"}
# Remove signature: header.payload.

# 2. Algorithm confusion (RS256 to HS256)
# If server uses RS256, try HS256 with public key as secret

# 3. Weak signing key
# Use hashcat or jwt-cracker to brute-force HMAC secret
hashcat -m 16500 jwt.txt wordlist.txt

# 4. Expiration bypass
# Modify "exp" claim to future timestamp
```

**Opaque Token Analysis:**
```
- Test token length and entropy
- Check if tokens are sequential/predictable
- Test token reuse after logout
- Verify token invalidation on password change
```

### Step 3: Test Authentication Bypass

```bash
# Test missing authentication
curl -X GET https://api.target.com/api/v1/users/profile

# Test with empty/null token
curl -X GET https://api.target.com/api/v1/users/profile \
  -H "Authorization: Bearer "

curl -X GET https://api.target.com/api/v1/users/profile \
  -H "Authorization: Bearer null"

# Test with expired token (should fail)
curl -X GET https://api.target.com/api/v1/users/profile \
  -H "Authorization: Bearer <expired_token>"

# Test token from different user
curl -X GET https://api.target.com/api/v1/users/123/profile \
  -H "Authorization: Bearer <user_456_token>"
```

### Step 4: Test IDOR / Broken Object-Level Authorization

```bash
# Change user ID in request path
curl -X GET https://api.target.com/api/v1/users/123/orders \
  -H "Authorization: Bearer <user_456_token>"

# Change object ID in request body
curl -X PUT https://api.target.com/api/v1/orders/789 \
  -H "Authorization: Bearer <user_456_token>" \
  -d '{"status": "cancelled"}'

# Test horizontal privilege escalation
# Access admin endpoints with regular user token
curl -X GET https://api.target.com/api/v1/admin/users \
  -H "Authorization: Bearer <regular_user_token>"
```

### Step 5: Test Session Management

```bash
# Test concurrent sessions
# Login from multiple devices simultaneously - should both remain valid?

# Test session invalidation after logout
TOKEN=$(curl -s -X POST https://api.target.com/api/v1/auth/login \
  -d '{"email":"test@test.com","password":"pass"}' | jq -r '.token')

# Logout
curl -X POST https://api.target.com/api/v1/auth/logout \
  -H "Authorization: Bearer $TOKEN"

# Try using the same token (should fail)
curl -X GET https://api.target.com/api/v1/users/me \
  -H "Authorization: Bearer $TOKEN"

# Test session invalidation after password change
# Token obtained before password change should be invalidated
```

### Step 6: Test OAuth 2.0 / OIDC Mobile Flows

```bash
# Test for authorization code interception
# Check if PKCE (Proof Key for Code Exchange) is enforced
# Test with missing code_verifier parameter

# Test redirect URI manipulation
# Try custom scheme hijacking: myapp://callback
# Test with modified redirect_uri parameter

# Test scope escalation
# Request higher privileges than granted
```

## Key Concepts

| Term | Definition |
|------|-----------|
| **BOLA/IDOR** | Broken Object Level Authorization - accessing resources by changing identifiers without server-side authorization checks |
| **JWT** | JSON Web Token - self-contained authentication token with header, payload, and signature components |
| **PKCE** | Proof Key for Code Exchange - OAuth 2.0 extension preventing authorization code interception in mobile apps |
| **Token Refresh** | Mechanism for obtaining new access tokens using long-lived refresh tokens without re-authentication |
| **Session Fixation** | Attack where adversary sets a known session ID before victim authenticates, then hijacks the session |

## Tools & Systems

- **Burp Suite**: HTTP proxy for intercepting and modifying authentication requests
- **jwt_tool**: Python tool for testing JWT vulnerabilities (none algorithm, key confusion, claim manipulation)
- **Postman**: API testing client for crafting authentication requests
- **hashcat**: Password/JWT secret cracking tool for testing HMAC signing key strength
- **Autorize**: Burp Suite extension for automated authorization testing

## Common Pitfalls

- **Rate limiting masks issues**: API may rate-limit test requests. Use delays between requests and test from the tester's authorized perspective first.
- **Token in URL**: Some mobile APIs pass tokens in URL query parameters, exposing them in server logs and browser history. Flag as finding even if authorization works correctly.
- **Refresh token rotation**: Some APIs rotate refresh tokens on each use. If your test invalidates the refresh token, you may lock out your test account.
- **Mobile-specific OAuth**: Mobile apps use custom URI schemes for OAuth redirects, which can be intercepted by malicious apps registered for the same scheme.