CVE-2023-6859 Use after free in Firefox

What is CVE-2023-6859?

CVE-2023-6859 is a high-severity use after free vulnerability in Debian Debian Linux, published on 19 Dec 2023. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official description

A use-after-free condition affected TLS socket creation when under memory pressure. This vulnerability affects Firefox ESR < 115.6, Thunderbird < 115.6, and Firefox < 121.

Weakness: CWE-416

Which versions are affected by CVE-2023-6859?

It affects 4 products.

Products include Debian Debian Linux, Mozilla Firefox, Mozilla Firefox Esr, Mozilla Thunderbird.

How do I fix CVE-2023-6859?

Upgrade to a fixed version.

Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.

Is CVE-2023-6859 being exploited?

There is no known exploitation of CVE-2023-6859 in the wild and no public exploit code.

Other vulnerabilities in Debian Debian Linux

• CVE-2015-3306 ProFTPD Improper Access Control Exploited Critical 10.0
• CVE-2025-49113 RoundCube Webmail Deserialization of Untrusted Data Exploited Critical 9.9
• CVE-2026-24061 GNU InetUtils Argument Injection Exploited Critical 9.8
• CVE-2025-39682 Linux Kernel Improper Check for Unusual or Exceptional Conditions Exploited Critical 9.8
• CVE-2024-9680 Mozilla Firefox Use-After-Free Exploited Critical 9.8

All Debian Debian Linux vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as DEBIAN-CVE-2023-6859, GHSA-gx9r-x967-x29h, UBUNTU-CVE-2023-6859.