---
id: CVE-2025-1020
title: Memory safety bugs present in Firefox 134 and Thunderbird 134
summary: >-
  Memory safety bugs present in Firefox 134 and Thunderbird 134. 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: critical
cvss: 9.8
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H'
cwe:
  - CWE-787
  - CWE-787
vendor: mozilla
product: firefox
affected:
  - firefox < 135.0
  - 'thunderbird >= 131.0, < 135.0'
patched:
  - firefox 135.0
  - thunderbird 135.0
published: '2025-02-04'
updated: '2026-09-30'
sourceUpdated: '2026-09-30T19:10:01.007'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2025-1020'
references:
  - url: 'https://bugzilla.mozilla.org/buglist.cgi?bug_id=1939063%2C1942169'
    label: security@mozilla.org
  - url: 'https://www.mozilla.org/security/advisories/mfsa2025-07/'
    label: security@mozilla.org
  - url: 'https://www.mozilla.org/security/advisories/mfsa2025-11/'
    label: security@mozilla.org
tags:
  - nvd
epss: 0.0054
epssPercentile: 0.43267
ingestedAt: '2026-09-30T19:21:07.225Z'
---

## Overview

Memory safety bugs present in Firefox 134 and Thunderbird 134. 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 135 and Thunderbird 135.

## Affected

- `firefox < 135.0`
- `thunderbird >= 131.0, < 135.0`

## Remediation

Upgrade past the affected range:

- `firefox 135.0`
- `thunderbird 135.0`