CVE-2026-8959

Name: CVE-2026-8959
Creator: NVD / NIST
Published: 2026-05-19T14:16:52.28+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

CRITICALCVSS 9.6/10EPSS 0.42%

Last modified Jun 17, 2026

CVE-2026-8959 is a critical-severity vulnerability rated 9.6/10 on the CVSS scale. 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.. EPSS estimates a 0.42% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

9.6/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H

EPSS Probability

0.42%

33.4th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 140.11.0
Mozilla	Firefox	< 151.0.0
Mozilla	Thunderbird	< 140.11
Mozilla	Thunderbird	< 151.0.0

References

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

Timeline

Published: May 19, 2026
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2026-8959?

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.

How severe is CVE-2026-8959?

CVE-2026-8959 has a CVSS score of 9.6/10 (CRITICAL severity). The EPSS model estimates a 0.42% probability of exploitation in the next 30 days.

How do I fix CVE-2026-8959?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2026-8959?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST