Today, we'll be diving deep into a recently discovered vulnerability in Mozilla's Firefox browser, referred to as CVE-2023-4055. We'll be discussing the issue, analyzing code snippets, reviewing exploit details, and finally learning how to mitigate this vulnerability. This affects Firefox versions earlier than 116, Firefox ESR before 102.14, and Firefox ESR before 115.1.

Issue Summary

The core problem with this vulnerability lies in the way Firefox handles cookies, specifically when the number of cookies per domain surpasses the limit set in document.cookie. The actual cookie jar, which stores all cookies related to a particular domain, gets out of sync with the expected cookie jar state, potentially causing some cookies to be missing in action when requests are sent.

A representation of the issue

// Exceeding cookie limit
document.cookie = "cookies_1=value_1";
document.cookie = "cookies_n=value_n";

This issue might not seem severe at first glance; however, considering that cookies are responsible for tracking user sessions, authentication data, and preferences, the missing cookies could cause critical errors in applications or even expose sensitive information.

The vulnerability was initially reported and discussed in the following sources

1. Mozilla Foundation Security Advisory 2023-09 (MFSA2023-09):
2. CVE Details:
3. National Cybersecurity and Communications Integration Center (NCCIC) Advisory:

Exploit Details

As of now, no known exploits specifically target this issue publicly. Still, it's essential to understand the dangers of leaving this vulnerability unaddressed. An attacker can craft a deliberate attack to maximize the chances of bypassing security mechanisms or exposing user data.

Intercepting and manipulating network traffic to take advantage of the missing cookies.

While we don't have full details or a proof-of-concept exploit to share, this vulnerability's presence in widely utilized software like Firefox serves as a reminder that we must always stay vigilant and up-to-date with the latest security advisories.

Mitigation Measures

To safeguard against this vulnerability, Mozilla has released security updates that address the issue. Affected parties should update their Firefox installations to the following versions, as applicable:

1. Firefox 116 and later:
2. Firefox ESR 102.14 and later:
3. Firefox ESR 115.1 and later:

Additionally, users should consider practicing safe browsing habits, such as employing a reputable ad-blocker, using secure connections (HTTPS), and avoiding sketchy or unfamiliar websites.


In this post, we covered the CVE-2023-4055 vulnerability, which causes cookie jar inconsistencies when the number of cookies per domain is exceeded in Firefox. We discussed the potential implications and provided links to original references. We also discussed the exploit details and mitigation measures.

It's crucial to stay informed and keep your software up-to-date to ensure the security of your Firefox browsing sessions.


Published on: 08/01/2023 16:15:00 UTC
Last modified on: 08/09/2023 21:15:00 UTC