Technical Analysis: "Exploiting a PFSense XSS via CSRF - And what it means for Internet-facing applications"

Write-up by: John Hammond

Summary:

In this video, John Hammond delves into a vulnerability in PFSense, a popular open-source firewall and router software. The demonstration reveals how Cross-Site Scripting (XSS) can be exploited via Cross-Site Request Forgery (CSRF) to achieve administrative control over an internet-facing web application.

Technical Details:

Vulnerability Overview:

  1. XSS Basics:
  2. Stored XSS: The payload is stored on the target server, e.g., in a database or a configuration file.
  3. Reflected XSS: The payload is reflected off a web server, such as in an error message or search results.

  4. DOM-based XSS: The vulnerability exists in the client-side code rather than the server-side code.

  5. CSRF Basics:

  6. CSRF Concept: An attack where a user is tricked into executing unwanted actions on a web application in which they are authenticated.
  7. CSRF Tokens: Unique tokens used to validate that the request comes from the authenticated user session.

The Exploit Process:

  1. XSS via CSRF Attack Chain:
  2. Objective: Use CSRF to inject an XSS payload that executes in the context of an authenticated admin user on PFSense.

  3. Challenge: Crafting a CSRF payload that the admin user will unknowingly trigger by simply visiting a malicious page.

  4. Attack Execution:

  5. Step 1: Identifying an entry point in PFSense vulnerable to CSRF.
  6. Step 2: Crafting a malicious HTML form that, when auto-submitted, would execute a request to the vulnerable PFSense endpoint.
  7. Step 3: Including the XSS payload within the CSRF exploit to ensure that upon submission, the payload injects malicious JavaScript into PFSense.
  8. Step 4: Hosting the malicious HTML on an attacker-controlled domain and social engineering the admin to visit the page.
  9. Step 5: Upon visiting, the form is auto-submitted to the vulnerable PFSense instance, injecting the XSS payload into its context.
  10. Step 6: The XSS payload executes in the admin's browser, allowing attackers to hijack the session, extract cookies, and potentially take full administrative control.

Technical Insights:

  1. PFSense Configuration Gaps:
  2. Lack of proper input sanitization ensures that malicious scripts can be injected via input fields.

  3. Insufficient CSRF protection enables unverified requests to be executed by authenticated users.

  4. HTML Form Crafting:

  5. The form targets a specific endpoint in PFSense, crucial for a successful CSRF attack.

  6. Form fields are populated with malicious entries exploiting PFSense's weak validation.

  7. JavaScript Execution:

  8. Injected scripts run in the context of the targeted user's session.

  9. Attacks can manipulate DOM elements, issue AJAX requests, or perform session hijacking.

  10. Mitigation Strategies:

  11. Input Validation and Sanitization: Sanitize and validate all user inputs rigorously.
  12. CSRF Protection: Use tokens and validate them on every state-changing request.
  13. CSP Headers: Configure Content Security Policy (CSP) headers to restrict resources the browser is allowed to load.
  14. Regular Patch Management: Frequently update and patch systems to close known vulnerabilities.

Conclusion:

The demonstration by John Hammond highlights the critical intersection of XSS and CSRF vulnerabilities and their devastating potential when exploited together. This analysis underscores the necessity for rigorous input validation, complete CSRF protection, and continuous security oversight on internet-facing applications.

For the complete video walk-through, watch the full demonstration on YouTube here.