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

Severity: critical (CVSS 9.6 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-05-19 · updated 2026-10-14
Weakness: CWE-20

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.

Affected products

Mozilla Firefox: before 140.11.0; before 151.0.0 — fixed in 140.11.0, 151.0.0
Mozilla Thunderbird: before 140.11; before 151.0.0 — fixed in 140.11, 151.0.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 140.11.0, 151.0.0; Mozilla Thunderbird 140.11, 151.0.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/show_bug.cgi?id=2034754
https://www.mozilla.org/security/advisories/mfsa2026-46/
https://www.mozilla.org/security/advisories/mfsa2026-48/
https://www.mozilla.org/security/advisories/mfsa2026-50/
https://www.mozilla.org/security/advisories/mfsa2026-51/

Get alerted when CVE-2026-8959 or the next vulnerability affects your stack