CVE-2025-1012: Uncovering the Race Condition during Concurrent Delazification leading to Use-After-Free Vulnerability in Older Versions of Firefox and Thunderbird

In this in-depth post, we reveal a crucial vulnerability, CVE-2025-1012, involving a race condition that occurs during the concurrent delazification process, resulting in a potentially disastrous use-after-free scenario. This vulnerability affects earlier versions of Mozilla Firefox, Firefox ESR, and Thunderbird. Our goal is to provide a comprehensive understanding of the exploit by explaining the key concepts involved, examining the code snippets responsible for the vulnerability, and pointing out the mitigation measures in place.

Background: Understanding delazification and use-after-free

Before we delve into CVE-2025-1012, let's touch upon the concepts of delazification and use-after-free to provide context.

Delazification: Delazification is the process of converting a partially constructed, or 'lazy', object into a fully initialized object within a JavaScript engine. The reason behind this process is the optimization of resources, as it ensures that objects that are not actively being used do not consume resources, thus improving overall performance.

Use-after-free: A use-after-free vulnerability arises when a program continues to reference a memory location after it has been freed. This can result in information leaks or, even worse, malicious code execution.

The Vulnerability: Race condition during concurrent delazification

Mozilla's Firefox, Firefox ESR, and Thunderbird are exposed to the CVE-2025-1012 vulnerability, which hails from a concurrency issue with delazification. When multiple threads attempt to delazify the same object at the same time, a race condition occurs, leading to a use-after-free situation.

Code Snippet: Delving into the exploit

The following code snippet illustrates a simplified scenario highlighting the race condition during concurrent delazification:

function delazify(obj) {
  if (isObjectLazy(obj)) {
    var initObj = completeInitialization(obj);

    // Potential race condition occurs here
    memoryBarrier();

    setInitializedObject(obj, initObj);
  }
}

In the above example, if two threads call the delazify function concurrently, they may both execute completeInitialization for the same lazy object and attempt to set its initialized value. This creates a race condition whereby one of the initialized objects can be orphaned and marked for garbage collection while still in use, resulting in a use-after-free vulnerability.

More information about the vulnerability can be found in the official CVE entry

- CVE-2025-1012
- Mozilla Security Advisory

Mitigation Measures

The following updates and patches have been released to address this vulnerability in Firefox, Firefox ESR, and Thunderbird:

Thunderbird version 135 and later

Users of the affected software are encouraged to update their systems to the latest versions, as mentioned above, to safeguard themselves against any potential attacks exploiting this vulnerability.

Conclusion

As demonstrated through the CVE-2025-1012 vulnerability, thorough awareness of the various components involved in an application, such as delazification and use-after-free, is necessary to avoid potentially harmful situations. By shedding light on this race condition and equipping users with the information required to tackle the exploit head-on, the internet becomes a safer place for all.

Timeline

Published on: 02/04/2025 14:15:32 UTC
Last modified on: 02/06/2025 19:33:46 UTC