In this detailed analysis, we will examine a recently discovered vulnerability, CVE-2023-4578, that affects Firefox < 117, Firefox ESR < 115.2, and Thunderbird < 115.2. Specifically, the vulnerability lies in the mishandling of a Syntax Error within the JS::CheckRegExpSyntax function. This can potentially lead to a memory allocation issue, creating a security risk for users of the aforementioned software versions.

Code Snippet

The vulnerability occurs during the execution of the convertToRuntimeErrorAndClear function, which is called if a Syntax Error is set during the JS::CheckRegExpSyntax function. Here's a snippet of the code in question:

JS::CheckRegExpSyntax(JSContext* cx, ...) {
    if (!CompileTokenStream(...)) {
        // Syntax error detected
        convertToRuntimeErrorAndClear(cx, ...);

void convertToRuntimeErrorAndClear(JSContext* cx, ...) {
    if (exception.is<JS::OutOfMemory>()) {
        // If exception is a newly created Out of Memory exception
    } else {
        // Handle Syntax Error

Root Cause

The vulnerability originates from a situation in which a Syntax Error is detected, but the convertToRuntimeErrorAndClear function is called. The function attempts to allocate memory when none is available, causing a newly created Out of Memory exception to be mistakenly handled as a Syntax Error. The lack of proper exception handling can lead to unintended behavior and potential security risks.

Exploit Details

To exploit this vulnerability, an attacker would need to craft a malicious JavaScript file or website that triggers a Syntax Error during the JS::CheckRegExpSyntax function. The attacker could then take advantage of the incorrect memory allocation handling to perform a malicious action, potentially compromising the user's system or exfiltrating sensitive data.

Original References

- CVE-2023-4578 - NVD
- Mozilla Foundation Security Advisory


Users are advised to update their Firefox, Firefox ESR, and Thunderbird installations to the latest versions, which addresses this vulnerability. Specifically, upgrade to Firefox 117 or later, Firefox ESR 115.2 or later, and Thunderbird 115.2 or later.

In conclusion, CVE-2023-4578 is a notable memory allocation vulnerability caused by the mishandling of Syntax Errors within the JS::CheckRegExpSyntax function. By understanding the root cause, exploit details, and applying the appropriate mitigation measures, users and developers can protect their systems from potential attacks.


Published on: 09/11/2023 09:15:00 UTC
Last modified on: 09/14/2023 03:53:00 UTC