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Fencer Vulns
high 8.8

CVE-2021-43534 json

Mozilla developers and community members reported memory safety bugs present in Firefox 93 and Firefox ESR 91.2.

published 2021-12-08 4 y ago modified 2026-06-17 4 mo ago also EUVD-2021-30463

cvss
8.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.2%
EPSS 1.2% chance of exploitation in 30 days, percentile 69%
public exploits
0
none known

Summary

Mozilla developers and community members reported memory safety bugs present in Firefox 93 and Firefox ESR 91.2. 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 < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.

Patch / fix

  • Mozilla Firefox 94
  • Mozilla Thunderbird 91.3
  • Mozilla Firefox ESR 91.3
  • firefox-esr Debian 14, Debian 13, Debian 12 91.3.0esr-1
  • thunderbird Debian 14, Debian 13, Debian 12 1:91.3.0-1

debian advisories DEBIAN-CVE-2021-43534

  • 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-2021-43534

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 7 91.3.0-2.el7_9
  • thunderbird RHEL 8 91.3.0-2.el8_4
  • firefox RHEL 8 91.3.0-1.el8_4
  • firefox RHEL 7 91.3.0-1.el7_9
  • thunderbird RHEL 8.1 EUS 91.3.0-2.el8_1
  • thunderbird RHEL 8.2 EUS 91.3.0-2.el8_2
  • firefox RHEL 8.2 EUS 91.3.0-1.el8_2
  • firefox RHEL 8.1 EUS 91.3.0-1.el8_1

red hat advisories RHSA-2021:4133RHSA-2021:4132RHSA-2021:4134RHSA-2021:4605RHSA-2021:4130RHSA-2021:4607

  • thunderbird Rocky Linux 8 91.3.0-2.el8_4
  • thunderbird Rocky Linux 8 91.3.0-2.el8
  • firefox Rocky Linux 8 91.3.0-1.el8

rocky linux advisories RLSA-2021:4130RLSA-2021:4123

  • thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 91.3.2-r0

alpine advisories ALPINE-CVE-2021-43534

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-2021-43534 updated cvss 8.8

Mozilla: Memory safety bugs fixed in Firefox 94 and Firefox ESR 91.3

  • Red Hat Enterprise Linux 6 firefox, thunderbird
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux AppStream (v. 8) firefox 91.3.0-1.el8_4, thunderbird 91.3.0-2.el8_4
    fixed: vendor fix RHSA-2021:4123
  • Red Hat Enterprise Linux AppStream EUS (v. 8.1) firefox 91.3.0-1.el8_1, thunderbird 91.3.0-2.el8_1
    fixed: vendor fix RHSA-2021:4607
  • Red Hat Enterprise Linux AppStream EUS (v. 8.2) firefox 91.3.0-1.el8_2, thunderbird 91.3.0-2.el8_2
    fixed: vendor fix RHSA-2021:4605
  • Red Hat Enterprise Linux Client (v. 7) firefox 91.3.0-1.el7_9, thunderbird 91.3.0-2.el7_9 Red Hat Enterprise Linux Client Optional (v. 7) firefox 91.3.0-1.el7_9 Red Hat Enterprise Linux Server (v. 7) firefox 91.3.0-1.el7_9 Red Hat Enterprise Linux Server Optional (v. 7) firefox 91.3.0-1.el7_9, thunderbird 91.3.0-2.el7_9 Red Hat Enterprise Linux Workstation (v. 7) firefox 91.3.0-1.el7_9, thunderbird 91.3.0-2.el7_9 Red Hat Enterprise Linux Workstation Optional (v. 7) firefox 91.3.0-1.el7_9
    fixed: vendor fix RHSA-2021:4116

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

Exploit prediction

1.25% chance of exploitation in the next 30 days, higher than 69% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 8.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
NVD 2.0 6.8 AV:N/AC:M/Au:N/C:P/I:P/A:P

Affected products

  • Mozilla Firefox
  • Mozilla Thunderbird
  • Mozilla Firefox ESR

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.