CVE-2026-100787 Sandbox escape in the XUL component

Critical 9.6 Published 29 Sep 2026 Updated 1 Oct 2026

A sandbox escape in the XUL component allows attackers to execute arbitrary code outside the browser sandbox.

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-2026-100787?

CVE-2026-100787 is a critical 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

Sandbox escape in the XUL component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 153.4, and Firefox 157.

Weakness: CWE-20

Which versions are affected by CVE-2026-100787?

It affects 2 products.

Products include Mozilla Firefox, Mozilla Thunderbird.

How do I fix CVE-2026-100787?

Upgrade to a fixed version.

Is CVE-2026-100787 being exploited?

There is no known exploitation of CVE-2026-100787 in the wild and no public exploit code.

All Mozilla Firefox vulnerabilities

Sources

Also known as DEBIAN-CVE-2026-100787, GHSA-jrr8-c8gg-qvgr, UBUNTU-CVE-2026-100787.