In early 2022, a security vulnerability tracked as CVE-2022-22737 emerged in Mozilla’s popular browsers, Firefox and Firefox ESR, as well as the Thunderbird email client. This post will break down how this bug happened, why it’s dangerous, and what an attacker could do with it. We’ll use straightforward language, explain the exploit with example code, and point to further reading.

What Is CVE-2022-22737?

CVE-2022-22737 is a race condition vulnerability in how Firefox handles audio playback. Race conditions happen when two or more parts of a program try to access shared data at the same time in ways that shouldn’t happen.

Thunderbird: versions \< 91.5

This bug boils down to a problem when constructing audio sinks—the bits of code that handle audio output—while playing audio and closing browser windows at the same time. If triggered, this bug could cause a use-after-free: a nasty type of crash where the program tries to use memory it just released, potentially opening the door for remote code execution.

The audio sink object is created to output the sound.

- At the same time—either by user action or a script—the window or tab closes while the audio is still being set up or played.
- Because of a race condition, the audio sink may be destroyed (freed from memory) while other code is still using it.
- That’s the “use-after-free.” This gives attackers a chance to put something malicious in that freed memory spot and hijack the browser.

Exploit Walkthrough: From Bug to Crash

Let’s look at a simple proof-of-concept that would trigger the bug.

HTML Proof-of-Concept

<!DOCTYPE html>
<html>
  <body>
    <script>
      // Create an audio element and start playback
      let audio = new Audio();
      audio.src = "data:audio/wav;base64,UklGRhYAAABXQVZFZm10IBAAAAABAAEAIlYAAB9AAACABAAZGFYQAAAAA=";
      audio.loop = true;
      audio.play();

      // Rapidly close the window after playback starts
      setTimeout(() => {
        window.close();
      }, 50);    // Close after 50ms
    </script>
    <p>If your browser is vulnerable, this might crash or hang it.</p>
  </body>
</html>

The above code starts playing a tiny audio file.

- 50 milliseconds later (setTimeout), it closes the window – causing a race while the audio sink is still being constructed.

On Firefox \< 96 or ESR \< 91.5, this could lead to a crash. With careful timing, an attacker might be able to re-use the memory slot (use-after-free) to perform advanced exploits.

Why is “use-after-free” so dangerous?

- Attackers can sometimes carefully place their own code/data where the freed object used to be.
- When Firefox tries to use the freed audio sink, it unknowingly runs or accesses the attacker's malicious data.
- With advanced techniques, this could lead to remote code execution—running malware, exfiltrating data, or even escaping the browser sandbox.

Think of this like someone walking out of a hotel room and the door unlocking, but instead of housekeeping coming in, a burglar moves in, putting their stuff everywhere. The next guest walks in and—surprise!—it’s not a safe environment anymore.

Technical Details & References

The AudioSinkWrapper in the Firefox code base is responsible for audio output. If Shutdown() is called while another thread is still using the audio sink, the object might be destroyed and, if accessed again, results in a dangling pointer.

Code snippet from Mozilla's official bug report:

RefPtr<AudioSink> mAudioSink;

void Shutdown() {
    // Potential race: mAudioSink can be destroyed while playback is ongoing.
    mAudioSink = nullptr;
}

Reference

- Mozilla Bugzilla #174409 – Full technical details and patch.

How Was It Fixed?

The Mozilla team fixed this by ensuring proper synchronization: they made sure an audio sink can’t just vanish while something else is still using it. In programming, this is typically done with mutexes or locks to block one piece of code from interfering with another.

Patch summary:

Ensure all threads finish before freeing the object.

See the patch for details.

How To Stay Safe?

If you’re running:

Thunderbird \< 91.5

Update immediately! Most users will have auto-updates, but it's worth checking.

Race conditions are tricky to exploit reliably, but not impossible.

- Modern browsers do have mitigations like sandboxing and heap protections, but use-after-free bugs are still among the top ways attackers break in.
- Simple scenarios (playing audio in a tab, script rapidly closing windows) can surface dangerous vulnerabilities—never underestimate denial-of-service PoCs!

Further Reading

- CVE-2022-22737 on NVD
- Mozilla’s Bugzilla Entry
- Firefox Security Advisories
- Firefox Release Notes 96.

Summary

CVE-2022-22737 shows how a simple bug—in how a browser plays audio—can have major consequences. By racing the creation and destruction of audio sinks, attackers could trigger a use-after-free leading to a crash or, with enough skill, full code execution. If you haven’t already, update your Firefox or Thunderbird, and stay safe online.

Timeline

Published on: 12/22/2022 20:15:00 UTC
Last modified on: 12/29/2022 15:50:00 UTC