---
id: CVE-2026-8959
title: >-
  Sandbox escape due to incorrect boundary conditions in the Widget: Win32
  component
summary: >-
  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.
severity: critical
cvss: 9.6
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H'
cwe:
  - CWE-20
  - CWE-119
  - CWE-693
vendor: mozilla
product: firefox
affected:
  - firefox < 140.11.0
  - firefox < 151.0.0
  - thunderbird < 140.11
  - thunderbird < 151.0.0
patched:
  - firefox 151.0.0
  - thunderbird 151.0.0
published: '2026-05-19'
updated: '2026-10-14'
sourceUpdated: '2026-10-14T20:10:00.190'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-8959'
references:
  - url: 'https://bugzilla.mozilla.org/show_bug.cgi?id=2034754'
    label: security@mozilla.org
  - url: 'https://www.mozilla.org/security/advisories/mfsa2026-46/'
    label: security@mozilla.org
  - url: 'https://www.mozilla.org/security/advisories/mfsa2026-48/'
    label: security@mozilla.org
  - url: 'https://www.mozilla.org/security/advisories/mfsa2026-50/'
    label: security@mozilla.org
  - url: 'https://www.mozilla.org/security/advisories/mfsa2026-51/'
    label: security@mozilla.org
tags:
  - nvd
epss: 0.00449
epssPercentile: 0.37449
ingestedAt: '2026-10-14T20:53:12.317Z'
---

## Overview

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

- `firefox < 140.11.0`
- `firefox < 151.0.0`
- `thunderbird < 140.11`
- `thunderbird < 151.0.0`

## Remediation

Upgrade past the affected range:

- `firefox 151.0.0`
- `thunderbird 151.0.0`