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:

# 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

# 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

# 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

# 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

# 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

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.