In this long read post, we will be discussing the vulnerability CVE-2023-5171, which affects Firefox versions below 118, Firefox ESR versions below 115.3, and Thunderbird versions below 115.3. This issue occurs during the Ion compilation process, when a Garbage Collection could result in a use-after-free condition, allowing an attacker to write two NUL bytes and potentially cause an exploitable crash.

Original References


Exploit Details

The vulnerability is caused by a flaw in the Ion compilation process during Garbage Collection, which is responsible for managing memory allocation and deallocation within Mozilla's browser engine. By inducing a Garbage Collection, the attacker can trigger a use-after-free condition, where an object's memory is still used even after it has been freed. This opens up the possibility of writing two NUL bytes, which can cause the application to crash.

Here's a simplified representation of the issue

void foo() {
    int *ptr = new int[10];
    delete[] ptr;

    // Use-After-Free Vulnerability
    ptr[] = ; // Writing NUL byte
    ptr[1] = ; // Writing another NUL byte

This sample code illustrates the use of a pointer after it has been deleted (freed). Even though the memory has been deallocated, the pointer still holds the address of the freed object. As a result, writing data to this location can cause unexpected behavior and crashes, which can be exploited by a skilled attacker.

Why is it Dangerous?

An exploitable crash is considered dangerous because it can be used to execute arbitrary code on the user's system, potentially compromising the user's security and privacy. An attacker can craft a malicious webpage or email message which exploits this vulnerability to run malware or gain unauthorized access to sensitive information.

How to Protect Yourself

Updates are available for Firefox, Firefox ESR, and Thunderbird, which address this vulnerability. Mozilla has released Firefox version 118, Firefox ESR 115.3, and Thunderbird 115.3 to fix this issue. Users are strongly encouraged to update their software to the latest versions.

1. Firefox:
2. Firefox ESR:
3. Thunderbird:


CVE-2023-5171 is a potentially exploitable vulnerability affecting Firefox, Firefox ESR, and Thunderbird. The vulnerability involves a use-after-free condition during Ion compilation, which allows an attacker to write two NUL bytes and cause a crash. Updating to the latest software versions is highly recommended to mitigate the risk posed by this vulnerability.


Published on: 09/27/2023 15:19:00 UTC
Last modified on: 10/12/2023 02:52:00 UTC