CVE-2026-6771 Mitigation bypass in the DOM: Security component

Critical 9.8 Published 21 Apr 2026 Updated 27 May 2026

An attacker can bypass DOM security mitigations in Firefox and Thunderbird, potentially enabling further exploitation of the browser environment.

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

What is CVE-2026-6771?

CVE-2026-6771 is a critical vulnerability in Mozilla Firefox, published on 21 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Mitigation bypass in the DOM: Security component. This vulnerability was fixed in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10.

Weakness: CWE-288

Which versions are affected by CVE-2026-6771?

It affects 2 products.

Products include Mozilla Firefox, Mozilla Thunderbird.

How do I fix CVE-2026-6771?

Upgrade to a fixed version.

Is CVE-2026-6771 being exploited?

There is no known exploitation of CVE-2026-6771 in the wild and no public exploit code.

All Mozilla Firefox vulnerabilities

Sources

Also known as DEBIAN-CVE-2026-6771, GHSA-88fc-5m2g-g6q2, UBUNTU-CVE-2026-6771.