CVE-2026-8959 Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component

Critical 9.6 Published 19 May 2026 Updated 19 May 2026

Attackers can escape the browser sandbox via incorrect boundary conditions in the Win32 widget component, potentially executing arbitrary code.

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-2026-8959?

CVE-2026-8959 is a critical vulnerability in Mozilla Firefox, published on 19 May 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 due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Firefox 151, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11.

Weakness: CWE-693, CWE-20, CWE-119

Which versions are affected by CVE-2026-8959?

It affects 2 products.

Products include Mozilla Firefox, Mozilla Thunderbird.

How do I fix CVE-2026-8959?

Upgrade to a fixed version.

Is CVE-2026-8959 being exploited?

There is no known exploitation of CVE-2026-8959 in the wild and no public exploit code.

All Mozilla Firefox vulnerabilities

Sources

Also known as GHSA-3x92-cxc7-h2pw, UBUNTU-CVE-2026-8959.