---
id: CVE-2025-6436
title: Memory safety bugs present in Firefox 139 and Thunderbird 139
summary: >-
  Memory safety bugs present in Firefox 139 and Thunderbird 139. Some of these
  bugs showed evidence of memory corruption and we presume that with enough
  effort some of these could have been exploited to run arbitrary code. This
  vulnerabili…
severity: high
cvss: 8.1
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H'
cwe:
  - CWE-119
vendor: mozilla
product: firefox
affected:
  - firefox < 140.0
  - thunderbird < 140.0
patched:
  - firefox 140.0
  - thunderbird 140.0
published: '2025-06-24'
updated: '2026-09-30'
sourceUpdated: '2026-09-30T18:10:00.190'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2025-6436'
references:
  - url: >-
      https://bugzilla.mozilla.org/buglist.cgi?bug_id=1941377%2C1960948%2C1966187%2C1966505%2C1970764%2C1942930
    label: security@mozilla.org
  - url: 'https://www.mozilla.org/security/advisories/mfsa2025-51/'
    label: security@mozilla.org
  - url: 'https://www.mozilla.org/security/advisories/mfsa2025-54/'
    label: security@mozilla.org
tags:
  - nvd
epss: 0.04256
epssPercentile: 0.90739
ingestedAt: '2026-09-30T18:17:24.446Z'
---

## Overview

Memory safety bugs present in Firefox 139 and Thunderbird 139. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability was fixed in Firefox 140 and Thunderbird 140.

## Affected

- `firefox < 140.0`
- `thunderbird < 140.0`

## Remediation

Upgrade past the affected range:

- `firefox 140.0`
- `thunderbird 140.0`