---
id: CVE-2026-84639
title: 'thunderbird: Uninitialized memory in MIME parsing (CVE-2026-84639)'
summary: >-
  A flaw was found in Thunderbird. The Mozilla Foundation's Security Advisory
  describes the following issue:

  Triggering an error condition in certain MIME bodies would cause uninitialized

  memory to be used.
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H'
cvssSource: vendor
cwe: CWE-824
vendor: Red Hat
product: Red Hat Enterprise Linux AppStream (v. 9)
affected:
  - enterprise_linux 10
  - enterprise_linux 6
  - enterprise_linux 7
  - enterprise_linux 8
  - enterprise_linux_appstream_v_9
patched:
  - enterprise_linux_appstream_v_9
published: '2026-09-01'
updated: '2026-09-24'
sourceUpdated: '2026-09-24T03:29:10+00:00'
source: CSAF
sourceUrl: 'https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-84639.json'
references:
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-84639.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-84639'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2527113'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-84639'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-84639'
  - url: 'https://www.mozilla.org/security/advisories/mfsa2026-87/#CVE-2026-84639'
  - url: 'https://access.redhat.com/errata/RHSA-2026:70642'
  - url: 'https://bugzilla.mozilla.org/show_bug.cgi?id=2061087'
  - url: 'https://www.mozilla.org/security/advisories/mfsa2026-86/'
  - url: 'https://www.mozilla.org/security/advisories/mfsa2026-87/'
  - url: 'https://www.mozilla.org/security/advisories/mfsa2026-88/'
tags:
  - csaf
  - vex
  - red-hat
  - cve.org
epss: 0.00323
epssPercentile: 0.2269
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-09-02T18:48:36.137068Z'
scores:
  vendor: 7.5
  adp: 9.1
ingestedAt: '2026-09-17T19:26:25.326Z'
---

## Overview

A flaw was found in Thunderbird. The Mozilla Foundation's Security Advisory describes the following issue:
Triggering an error condition in certain MIME bodies would cause uninitialized
memory to be used.

## Vendor advisories

- **RHSA-2026:70642** · Red Hat · fixed in: Red Hat Enterprise Linux AppStream (v. 9) · released 2026-09-24 · [advisory](https://access.redhat.com/errata/RHSA-2026:70642)
- **Red Hat VEX** · Important · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8 · no fix planned: Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 8 · updated 2026-09-24 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-84639.json)

**thunderbird: Uninitialized memory in MIME parsing** — rated Important by Red Hat. Released 2026-09-01, updated 2026-09-24.

Affected:

- Red Hat Enterprise Linux 10
- Red Hat Enterprise Linux 6
- Red Hat Enterprise Linux 7
- Red Hat Enterprise Linux 8

Fixed:

- Red Hat Enterprise Linux AppStream (v. 9)

No fix planned:

- Red Hat Enterprise Linux 6
- Red Hat Enterprise Linux 7
- Red Hat Enterprise Linux 10
- Red Hat Enterprise Linux 8

## Remediation

For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258 https://access.redhat.com/errata/RHSA-2026:70642