The Common Vulnerabilities and Exposures (CVE) database has recently published a newly identified security vulnerability, CVE-2023-4577, affecting Mozilla Firefox, Firefox ESR, and Thunderbird. In this post, we'll dive deep into the details of this vulnerability, explain how it can potentially be exploited, and provide links to the original references.

CVE-2023-4577: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-4577

Vulnerability Details

The root cause of this vulnerability lies in the UpdateRegExpStatics function, specifically when it tries to access initialStringHeap, which could have already been garbage collected. This could lead to a crash that could be potentially exploited by a malicious attacker.

Thunderbird < 115.2

It's essential for users running these vulnerable versions to update their software to the latest versions available from Mozilla:

- Firefox: https://www.mozilla.org/en-US/firefox/new/
- Firefox ESR: https://www.mozilla.org/en-US/firefox/organizations/
- Thunderbird: https://www.thunderbird.net/en-US/

Here is a code snippet that showcases where the issue occurs

void UpdateRegExpStatics(JSContext* cx, HandleObject regexpStatics,
                         HandleString input, RegExpRunStatus status) {
  // ... code omitted ...
  // Access initialStringHeap without ensuring it hasn't been garbage collected
  Rooted<JSFlatString*> flat(cx, initialStringHeap->getFlat(str));

  // ... code omitted ...

In this code snippet, the initialStringHeap variable is accessed directly without ensuring it hasn't been garbage collected. This means before the UpdateRegExpStatics function call, the memory stored in the initialStringHeap could have been released, causing the crash.

Exploiting the Vulnerability

To exploit this vulnerability, an attacker could craft a malicious webpage containing JavaScript code that takes advantage of the improper access of initialStringHeap. By doing this, the attacker could potentially execute arbitrary code on victims' machines or cause denial-of-service (DoS) attacks when the victim visits the webpage.

Let's assume an attacker crafts a sample malicious JavaScript code as follows

// hypothetical code demonstrating the exploitation
function triggerVulnerability() {
  // Craft an appropriate RegExp pattern and input string, then call UpdateRegExpStatics function

// Lure victim to visit webpage containing the above code

Visiting a webpage containing such code could lead to instability, crashes, or potentially, the execution of arbitrary code on the victim's machine.


The CVE-2023-4577 vulnerability discovered in Firefox, Firefox ESR, and Thunderbird exposes potential threats owing to potential garbage collection mishandling. To ensure security, users must update their Mozilla software to the latest versions.

Stay safe, stay updated, and keep your software patched to prevent exploitation.


Published on: 09/11/2023 09:15:00 UTC
Last modified on: 09/13/2023 03:46:00 UTC