Detailed Technical Analysis of "CVE-2024-4367: Arbitrary JS Execution in PDF.js"

Overview:
This write-up by Codean Labs provides an in-depth analysis of a critical security vulnerability (CVE-2024-4367) in PDF.js, a widely used web-based PDF viewer. The vulnerability permits arbitrary JavaScript execution, posing significant risks for various web applications embedding PDF.js.

Key Technical Details:

1. Vulnerability Description:
   The vulnerability stems from improper handling of JavaScript actions embedded within PDF files by PDF.js. Specifically, the issue is rooted in the way PDF.js processes and executes JavaScript actions. These actions are defined using PDF annotations or interactive forms which can include embedded JavaScript.

2. PDF Actions:
   PDF format includes actions that can trigger JavaScript code. These are typically found in annotations (like links, buttons) and forms. When a user interacts with such elements in a PDF, the associated JavaScript is executed.

3. Exploit Mechanism:
   Attackers can craft malicious PDFs containing JavaScript that takes advantage of this improper handling to execute arbitrary code in the context of the application viewing the PDF:

4. Annotation objects: Annotations can contain /Action or /A entries that specify JavaScript to be executed.
5. Open Action: The /OpenAction field in the PDF catalog can be used to automatically execute JavaScript when the document is opened.

Example: ```pdf << /OpenAction << /Type /Action /S /JavaScript /JS (alert('Executed via CVE-2024-4367')) >>

```

1. Security Flaws in PDF.js: PDF.js fails to properly sanitize or restrict the execution environment of JavaScript actions. This allows the injection and execution of malicious scripts, breaching security policies such as the same-origin policy.

2. Impact:
   This flaw enables:

3. Cross-Site Scripting (XSS): If a server hosts user-uploaded PDFs and renders them using PDF.js, an attacker could inject a malicious PDF to execute scripts in the context of the user's session.
4. Potential Data Theft: Access to session cookies, tokens, or other sensitive data.

5. Further Escalation: Depending on the privileges, malware deployment or deeper system compromise.

6. Mitigation Steps: Codean Labs suggests the following mitigation strategies:

7. Patching: Updating to a newer version of PDF.js where the vulnerability is fixed.
8. Content Security Policies (CSP): Strengthening CSPs to restrict script execution origin.
9. PDF Validation: Employing robust sanitization and validation of PDFs before rendering.
10. Browser-Level Mitigation: Advocating for browser defenses that restrict the execution of potentially malicious scripts from PDFs.

Exploit Example:

• Crafting a Malicious PDF:
  Attackers can embed JavaScript within an annotation:

1 0 obj
<<
  /Type /Annot
  /Subtype /Text
  /Contents (Click Me)
  /A <<
    /S /JavaScript
    /JS (alert('Arbitrary JS Execution'))
  >>
>>
endobj

• Deploying the PDF:
  Upload the malicious PDF to a site utilizing PDF.js for rendering. Upon opening or interacting with the PDF, the embedded JavaScript executes.

Key Takeaways:

• JavaScript in PDFs: PDF format supports JavaScript, used in forms and annotations, which can be exploited if not properly sandboxed.
• Sanitization & Validation: Critical for preventing arbitrary code execution within trusted environments.
• PDF.js: Popularity and widespread use mean that vulnerabilities can have extensive implications, pressing the need for continuous security audits and updates.

Conclusion:

The reported vulnerability, CVE-2024-4367, underlines the importance of thorough input validation and sanitization in applications processing complex file formats like PDFs. Users of PDF.js are urged to update to the latest versions and implement additional security measures as prescribed. This vulnerability showcases typical pitfalls in handling dynamic content within static files and the continuous evolving nature of web security threats.

For full details, check the original blog post here.