What is CVE-2026-8959?

CVE-2026-8959 is a critical-severity vulnerability in Mozilla Firefox, classified under Improper Input Validation. CVSS score: 9.6/10. Published 2026-05-19.

How severe is CVE-2026-8959?

Critical severity. CVSS v3 base score is 9.6 out of 10.

Buffer overflow in Mozilla Firefox

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.

Vulnerability class: Drupalgeddon 2 (CVE-2018-7600)

Published 2026-05-19 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.6 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.004 (36.6th percentile), scored 2026-09-30.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 37th percentile — 36.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.004 · newest: 0.004 · change: +0.000

Affected products

Mozilla Firefox — versions 140.11, 151
Mozilla Thunderbird — versions 140.11, 151

Weakness classification (CWE)

References

bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
www.mozilla.org/security/advisories/mfsa2026-46/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-48/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-50/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-51/ (Vendor Advisory)

Frequently asked questions

What is CVE-2026-8959?: CVE-2026-8959 is a critical-severity vulnerability in Mozilla Firefox, classified under Improper Input Validation. CVSS score: 9.6/10. Published 2026-05-19.
How severe is CVE-2026-8959?: Critical severity. CVSS v3 base score is 9.6 out of 10.