CVE-2026-100831 Use-after-free in the DOM: UI Events & Focus Handling component

High 8.8 Published 29 Sep 2026 Updated 30 Sep 2026

A use-after-free flaw in Firefox's DOM event handling allows attackers to execute arbitrary code, requiring users to update their browsers.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: A user has to click or open something
Fix: Available

What is CVE-2026-100831?

CVE-2026-100831 is a high-severity use after free vulnerability in Mozilla Firefox, published on 29 Sep 2026. 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

Use-after-free in the DOM: UI Events & Focus Handling component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 153.4, and Firefox 157.

Weakness: CWE-416

Which versions are affected by CVE-2026-100831?

It affects 2 products.

Products include Mozilla Firefox, Mozilla Thunderbird.

How do I fix CVE-2026-100831?

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-2026-100831 being exploited?

There is no known exploitation of CVE-2026-100831 in the wild and no public exploit code.

All Mozilla Firefox vulnerabilities

Sources

Also known as DEBIAN-CVE-2026-100831, GHSA-j53f-3p69-9c6v, UBUNTU-CVE-2026-100831.