Skip to content
Fencer Vulns
critical 9.8

CVE-2021-4129 json

Mozilla developers and community members Julian Hector, Randell Jesup, Gabriele Svelto, Tyson Smith, Christian Holler, and Masayuki Nakano…

published 2022-12-22 3 y ago modified 2026-06-17 4 mo ago also EUVD-2021-34003

cvss
9.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
1.1%
EPSS 1.1% chance of exploitation in 30 days, percentile 64%
public exploits
0
none known

Summary

Mozilla developers and community members Julian Hector, Randell Jesup, Gabriele Svelto, Tyson Smith, Christian Holler, and Masayuki Nakano reported memory safety bugs present in Firefox 94. 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 affects Firefox < 95, Firefox ESR < 91.4.0, and Thunderbird < 91.4.0.

Patch / fix

  • Mozilla Firefox 95
  • Mozilla Firefox ESR 91.4.0
  • Mozilla Thunderbird 91.4.0
  • firefox-esr Debian 14, Debian 13, Debian 12 91.4.0esr-1
  • thunderbird Debian 14, Debian 13, Debian 12 1:91.4.0-1

debian advisories DEBIAN-CVE-2021-4129

  • thunderbird Ubuntu 18.04 LTS 1:91.5.0+build1-0ubuntu0.18.04.1
  • thunderbird Ubuntu 20.04 LTS 1:91.5.0+build1-0ubuntu0.20.04.1

ubuntu advisories UBUNTU-CVE-2021-4129

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • thunderbird RHEL 8 91.4.0-2.el8_5
  • thunderbird RHEL 7 91.4.0-3.el7_9
  • firefox RHEL 7 91.4.0-1.el7_9
  • firefox RHEL 8 91.4.0-1.el8_5
  • thunderbird RHEL 8.1 E4S 91.4.0-2.el8_1
  • thunderbird RHEL 8.2 EUS 91.4.0-2.el8_2
  • thunderbird RHEL 8.4 EUS 91.4.0-2.el8_4
  • firefox RHEL 8.1 E4S 91.4.0-1.el8_1

red hat advisories RHSA-2021:5055RHSA-2021:5047RHSA-2021:5048RHSA-2021:5045RHSA-2021:5046RHSA-2021:5017

  • thunderbird Rocky Linux 8 91.4.0-2.el8_5
  • firefox Rocky Linux 8 91.4.0-1.el8_5

rocky linux advisories RLSA-2021:5045RLSA-2021:5013

  • thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 91.4.0-r0

alpine advisories ALPINE-CVE-2021-4129

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-4129 updated cvss 9.8

Mozilla: Memory safety bugs fixed in Firefox 95 and Firefox ESR 91.4

  • Red Hat Enterprise Linux 6 firefox, thunderbird
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux AppStream (v. 8) firefox 91.4.0-1.el8_5, thunderbird 91.4.0-2.el8_5
    fixed: vendor fix RHSA-2021:5013
  • Red Hat Enterprise Linux AppStream E4S (v. 8.1) firefox 91.4.0-1.el8_1, thunderbird 91.4.0-2.el8_1
    fixed: vendor fix RHSA-2021:5017
  • Red Hat Enterprise Linux AppStream EUS (v. 8.2) firefox 91.4.0-1.el8_2, thunderbird 91.4.0-2.el8_2
    fixed: vendor fix RHSA-2021:5016
  • Red Hat Enterprise Linux AppStream EUS (v.8.4) firefox 91.4.0-1.el8_4, thunderbird 91.4.0-2.el8_4
    fixed: vendor fix RHSA-2021:5015
  • Red Hat Enterprise Linux Client (v. 7) firefox 91.4.0-1.el7_9, thunderbird 91.4.0-3.el7_9 Red Hat Enterprise Linux Client Optional (v. 7) firefox 91.4.0-1.el7_9 Red Hat Enterprise Linux Server (v. 7) firefox 91.4.0-1.el7_9 Red Hat Enterprise Linux Server Optional (v. 7) firefox 91.4.0-1.el7_9, thunderbird 91.4.0-3.el7_9 Red Hat Enterprise Linux Workstation (v. 7) firefox 91.4.0-1.el7_9, thunderbird 91.4.0-3.el7_9 Red Hat Enterprise Linux Workstation Optional (v. 7) firefox 91.4.0-1.el7_9
    fixed: vendor fix RHSA-2021:5014

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
total
Assessed
18 mo ago

Exploit prediction

1.06% chance of exploitation in the next 30 days, higher than 64% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • bugzilla.mozilla.org https://bugzilla.mozilla.org/buglist.cgi?bug_id=1393362%2C1736046%2C1736751%2C1737009%2C1739372%2C1739421 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CISA ADP 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CNA 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected products

  • Mozilla Firefox
  • Mozilla Firefox ESR
  • Mozilla Thunderbird

Classification

Weakness

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.