CVE-2026-8959CWE-119CWE-20CWE-693

CVE-2026-8959

Critical · published May 19, 2026

CVSS v3.1

9.6

EPSS

Percentile

38.0

In the wild

Unconfirmed

What it is

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.

The record

Technical detail

CVSS v3.1: 9.6 · CRITICAL
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H
CVSS v4.0: Not supplied
EPSS: 0.00449 · 38.0th percentile
Weaknesses: CWE-119 · Improper Restriction of Operations within the Bounds of a Memory Buffer; CWE-20 · Improper Input Validation; CWE-693 · Protection Mechanism Failure
Published: 2026-05-19T18:16Z

Affected products (4)

Product	Versions	Fixed in
mozilla/firefox	< 140.11.0	140.11.0
mozilla/firefox	< 151.0.0	151.0.0
mozilla/thunderbird	< 140.11	140.11
mozilla/thunderbird	< 151.0.0	151.0.0

References (5)

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

EPSS history

Timeline

19 MAY 12:29Z
Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component
cvelistv5