XSS via Image/Video Files

TL;DR

Yes, corrupted image or video files can be exploited for Cross-Site Scripting (XSS) attacks. This happens because some applications try to process these files even when they’re broken, and vulnerabilities in that processing can allow malicious code to run.

How it Works

Applications often handle image/video files by attempting to read metadata or render them. If the application doesn’t properly sanitise data during this process, an attacker can craft a file containing malicious JavaScript code which will then be executed in a user’s browser.

Steps to Understand and Mitigate

1. Understand the Vulnerability: Many image/video libraries (like ImageMagick, FFmpeg) have known vulnerabilities related to parsing malformed files. These can lead to code execution or XSS if not patched.
2. Identify Potential Entry Points: Look for places in your application where users upload images/videos. This includes profile pictures, attachments, content management systems (CMS), and any other feature that accepts file uploads.
3. File Type Validation: Never rely solely on the file extension.
   • Check Magic Numbers: Verify the file type based on its magic number (the first few bytes of the file). This is a more reliable method than checking the extension.

     # Example using 'file' command in Linux:
     file myfile.jpg

   • Whitelist Allowed Types: Only allow specific, necessary image/video types (e.g., JPEG, PNG, MP4). Reject anything else.
4. Metadata Sanitisation: If you extract metadata from images/videos (EXIF data, tags, etc.), carefully sanitise it before displaying or using it.
   • Remove HTML Tags: Strip any HTML tags from the metadata.

     # Example in PHP:
     $metadata = strip_tags($metadata);

   • Encode Special Characters: Encode characters that have special meaning in HTML (e.g., <, >, &, “, ‘).

     # Example in PHP:
     $metadata = htmlspecialchars($metadata, ENT_QUOTES, 'UTF-8');

5. Image/Video Processing Libraries:
   • Keep Libraries Updated: Regularly update your image/video processing libraries to the latest versions to benefit from security patches.
   • Use Safe Rendering Methods: If possible, use rendering methods that don’t involve executing arbitrary code.
     For example, consider using a library’s built-in sanitisation functions or a sandboxed environment for image/video processing.
6. Content Security Policy (CSP): Implement a strong CSP to restrict the sources from which scripts can be loaded and executed.

   # Example CSP header:
   Content-Security-Policy: default-src 'self'; script-src 'self' 'unsafe-inline' https://example.com; img-src 'self' data:;

7. Input Length Limits: Limit the maximum file size and dimensions of uploaded images/videos to prevent denial-of-service attacks and reduce the complexity of processing.
8. Regular Security Audits: Conduct regular security audits and penetration testing to identify potential vulnerabilities in your application.

Example Attack Scenario

An attacker crafts a JPEG file containing malicious JavaScript code within its metadata. When the application processes this file, it extracts the metadata without proper sanitisation and displays it on a webpage. The malicious script then executes in the user’s browser.

Resources

• OWASP: Top 10 vulnerabilities
• SANS Institute: cyber security resources