In October 2023, Mozilla published a critical security advisory about a vulnerability in its flagship products Firefox, Firefox ESR, and Thunderbird. The issue is tracked as CVE-2023-5728. At the heart of this bug is a mishap during garbage collection, where extra operations were incorrectly performed on objects that should no longer be manipulated. This seemingly minor slip could lead to a dangerous crash scenario — and under the hood, open the door to remote code execution.

In this article, we will break down the vulnerability in simple American English, walk through how garbage collection is involved, look at the patch, share a proof-of-concept crash trigger, and reference the main sources.

What Is The Problem?

When a web browser like Firefox runs JavaScript, it creates lots of objects in memory. To keep this memory from getting clogged, the browser uses something called a garbage collector: a system that automatically gets rid of objects that are no longer needed.

But what happens if the garbage collector *misfires* and tries to clean an object it shouldn’t — or worse, tries to use an object after it’s already been cleaned up?

That’s exactly what happened in CVE-2023-5728.

A flaw in Mozilla’s garbage collector meant that, in specific situations, the software would try to perform operations on an object that had already been “collected.” This could corrupt memory, trigger a crash, and potentially allow an attacker to exploit the browser (for example, running malicious code in your session).

Sweep phase: Get rid of objects that aren’t marked — they’re no longer needed.

The problem in CVE-2023-5728: During the sweep, some extra code could still operate on certain already-freed objects — a classic Use-After-Free (UAF) scenario.

Exploit Scenario

Suppose a malicious website crafts JavaScript that triggers the garbage collector at just the right moment. If they can cause a UAF like this, they may:

Sometimes, force the browser to run attacker-chosen code (most dangerous).

A crash alone is a warning, but running attacker code means the vulnerability is critical.

Code Snippet: Triggering a Crash

Here’s a simplified JavaScript snippet that *could* help trigger garbage collector bugs (for educational purposes):

function trigger_gc_crash() {
  let objects = [];
  for (let i = ; i < 100000; i++) {
    objects.push({a: i});
  }
  // Remove strong references
  objects = null;
  // Force garbage collection (non-standard, but in debug builds this works)
  if (window.gc) { 
    gc();
  }
}

// Fast mutation of DOM objects (tricks garbage collector)
for (let i = ; i < 10000; i++) {
  let div = document.createElement('div');
  document.body.appendChild(div);
  document.body.removeChild(div);
}

// Try to crash with the collector
trigger_gc_crash();

Note: Modern browsers restrict user access to the gc() function — it’s usually only in special debug builds. But attackers often find side paths to force garbage collection indirectly.

The Patch

Mozilla's developers patched this vulnerability by better tracking which objects are safe to operate on during garbage collection. They made sure that once an object is freed, no further operations can happen on it.

You can see the relevant fix as part of Bugzilla bug 1859377 (private for now, but here’s the stub):

A direct code diff is not available right now. However, the heart of the fix is that it prevents any operation on objects once they are slated for collection.

Detailed References

- Mozilla Foundation Security Advisory 2023-44 (official)
- NVD - CVE-2023-5728 Summary
- Bugzilla bug #1859377 (requires permission)
- Firefox Release Notes 119
- Thunderbird Release Notes 115.4.1

If you manage enterprise machines: Deploy the latest ESR.

- Cautious browsing: While the bug is a deep technical one, its most likely exploitation would be via malicious websites.

Final Thoughts

CVE-2023-5728 teaches us that even mature browsers can stumble on memory management, putting millions at risk. Thanks to the quick action of Mozilla engineers, the window for exploitation was rapidly closed.

Stay updated, and remember: behind every browser update, there’s a hidden story of expert bug-hunting.


Disclaimer: This post is for educational purposes only and does not provide any new exploit techniques. Always update your software and browse safely!

Timeline

Published on: 10/25/2023 18:17:44 UTC
Last modified on: 11/02/2023 20:12:56 UTC