CVE-2026-2771 Undefined behavior in the DOM: Core & HTML component

Critical 9.8 Published 24 Feb 2026 Updated 21 Sep 2026

Undefined behavior in the DOM component allows attackers to cause memory corruption, potentially leading to code execution or crashes.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-2771?

CVE-2026-2771 is a critical memory corruption vulnerability in Mozilla Firefox, published on 24 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Undefined behavior in the DOM: Core & HTML component. This vulnerability was fixed in Firefox 148, Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird 148, and Thunderbird 140.8.

Weakness: CWE-125

Which versions are affected by CVE-2026-2771?

It affects 2 products.

Products include Mozilla Firefox, Mozilla Thunderbird.

How do I fix CVE-2026-2771?

Upgrade to a fixed version.

Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.

Is CVE-2026-2771 being exploited?

There is no known exploitation of CVE-2026-2771 in the wild and no public exploit code.

All Mozilla Firefox vulnerabilities

Sources

Also known as DEBIAN-CVE-2026-2771, GHSA-m65f-px5x-xq9x, UBUNTU-CVE-2026-2771.