A critical security flaw was found and fixed in PDF.js, the popular JavaScript library used by Firefox for rendering PDF files. Tracked as CVE-2024-4367, this vulnerability results from a missing type check when processing fonts in PDF files. Hackers can exploit this bug to execute arbitrary JavaScript code in the context of PDF.js, affecting Firefox < 126, Firefox ESR < 115.11, and Thunderbird < 115.11. This article breaks down how the bug works, shows what the vulnerable code looks like, and demonstrates how an attacker might exploit it.

What is the Issue?

PDF.js was not properly verifying the data type of objects related to font handling when rendering PDFs. This allowed a specially crafted PDF file to inject and execute JavaScript code. If a user opened a malicious PDF, an attacker could hijack the PDF.js environment, steal data, or even run further attacks in the browser sandbox.

Technical Details

When a font is loaded from a PDF, PDF.js expects the data to be a valid font dictionary object. Before CVE-2024-4367, PDF.js didn't check that the object was actually of the right type.

Here's a simplified snippet (not actual code) showing where the mistake happened

// Vulnerable font processing in PDF.js
function processFont(fontObject) {
  // No check if fontObject is actually an object or has expected props!
  let fontData = fontObject.data;

  // unsafe usage: attacker controls fontObject, could be anything!
  doSomethingWithFont(fontData);
}

What's missing? There should be a check that fontObject is indeed a valid object and not a function or other data type. An attacker could inject a function or malicious object.

Corrected code might look like

function processFont(fontObject) {
  if (typeof fontObject !== "object" || fontObject === null) {
    throw new Error("Invalid font object");
  }
  // Additional type checks here
  let fontData = fontObject.data;
  doSomethingWithFont(fontData);
}

How Could an Attacker Exploit This?

An attacker creates a malicious PDF that includes a specially crafted font resource. When Firefox or Thunderbird with a vulnerable PDF.js version opens the file, the font handler calls the attacker's injected JavaScript code.

Let's imagine a malicious PDF includes a font definition like this (simplified for illustration)

// In a crafted PDF, fontObject becomes a function!
let fontObject = function() { 
  // Malicious code here
  alert('CVE-2024-4367 exploited!'); 
};
processFont(fontObject); // Will execute the function!

When processFont uses fontObject, it’s actually running arbitrary JavaScript controlled by the attacker.

Stealing user data: For example, extracting sensitive information from the rendered PDF viewer.

- Further attacks: The attacker could access or manipulate browser features within the PDF.js sandbox.
- Persistent threats: If combined with other browser vulnerabilities, it could lead to larger scale browser or even OS-level compromise.

References and Further Reading

- Mozilla Foundation Security Advisory 2024-XXXX
- Bugzilla entry for CVE-2024-4367
- PDF.js GitHub Repository
- CVE-2024-4367 at NVD

Conclusion

CVE-2024-4367 is a powerful reminder that seemingly small type checks are crucial for application security, especially when parsing complex formats like PDF. If your system uses Firefox or Thunderbird, be sure to update today. For developers, always validate inputs—don’t trust your code’s assumptions!


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Timeline

Published on: 05/14/2024 18:15:12 UTC
Last modified on: 01/22/2025 17:16:51 UTC