Mastering OAuth 20 Flows: Complete Guide + Security Testing Tips

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OAuth 2.0 is a critical framework for authorization, widely used in modern web applications. Understanding its flows—Implicit, Authorization Code, Device Code, PKCE, and OpenID Connect—is essential for security professionals. Below, we break down each flow, common vulnerabilities, and how to test them.

1. Implicit Flow

  • Used for client-side apps (e.g., SPAs).
  • Vulnerability: Access tokens exposed in URLs (token leakage).
  • Testing Command:
    curl -I "https://example.com/oauth?response_type=token&client_id=CLIENT_ID&redirect_uri=CALLBACK_URL"

Check for tokens in browser history or logs.

2. Authorization Code Flow

  • Most secure flow (uses back-end exchange).
  • Vulnerability: CSRF attacks during code exchange.
  • Testing Command:
    curl -X POST "https://example.com/token" -d "grant_type=authorization_code&code=AUTH_CODE&redirect_uri=CALLBACK_URL"

Verify PKCE enforcement.

3. Device Code Flow

  • For IoT/limited-input devices.
  • Vulnerability: Slow polling attacks.
  • Testing Command:
    curl -X POST "https://example.com/device_authorize" -d "client_id=CLIENT_ID&scope=openid"

Monitor for excessive token requests.

4. PKCE (Proof Key for Code Exchange)

  • Mitigates authorization code interception.
  • Vulnerability: Weak code_verifier entropy.
  • Testing Command:
    openssl rand -hex 32 | sha256sum | cut -d ' ' -f1

Ensure `code_challenge` matches.

5. OpenID Connect (OIDC)

  • Extends OAuth for identity verification.
  • Vulnerability: ID token tampering.
  • Testing Command:
    jwt_tool <JWT_TOKEN> -T

Check for weak signing (e.g., `none` algorithm).

You Should Know:

  • Token Validation: Always validate tokens server-side:
    curl -H "Authorization: Bearer ACCESS_TOKEN" https://api.example.com/data
  • Log Analysis: Use `grep` to detect OAuth misconfigs:
    grep "response_type=token" /var/log/nginx/access.log
  • Rate Limiting: Prevent brute-forcing with iptables:
    iptables -A INPUT -p tcp --dport 443 -m connlimit --connlimit-above 50 -j DROP

What Undercode Say:

OAuth 2.0 is powerful but risky if misconfigured. Always enforce PKCE, validate tokens, and monitor logs. Use tools like `Burp Suite` and `jwt_tool` for testing.

Expected Output:

  • Secure OAuth implementation.
  • Logs free from token leaks.
  • Mitigated CSRF and brute-force attacks.

URL: Mastering OAuth 2.0 Flows – YouTube

Prediction: OAuth 2.0 adoption will grow, but so will attacks targeting weak implementations—automated scanning tools will become essential.

IT/Security Reporter URL:

Reported By: Insha J – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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