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Fencer Vulns
medium 4.3

CVE-2021-43538 json

By misusing a race in our notification code, an attacker could have forcefully hidden the notification for pages that had received full scr…

published 2021-12-08 4 y ago modified 2026-06-17 4 mo ago also EUVD-2021-30467

cvss
4.3
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 68%
public exploits
0
none known

Summary

By misusing a race in our notification code, an attacker could have forcefully hidden the notification for pages that had received full screen and pointer lock access, which could have been used for spoofing attacks. This vulnerability affects Thunderbird < 91.4.0, Firefox ESR < 91.4.0, and Firefox < 95.

Patch / fix

  • Mozilla Thunderbird 91.4.0
  • Mozilla Firefox ESR 91.4.0
  • Mozilla Firefox 95
  • 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-2021-43538

  • firefox Ubuntu 18.04 LTS 95.0+build1-0ubuntu0.18.04.1
  • firefox Ubuntu 20.04 LTS 95.0+build1-0ubuntu0.20.04.1
  • firefox Ubuntu 22.04 LTS 95.0+build1-0ubuntu1
  • 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-2021-43538

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

  • MozillaThunderbird SLE Workstation Extension 15 SP3, SLE Workstation Extension 15 SP2 91.4.0-8.45.2
  • MozillaFirefox SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +11 more 91.4.0-112.83.1
  • MozillaFirefox SLES for SAP 15 SP1, SLES for SAP 15, SLES 15-LTSS +7 more 91.4.0-150.9.1
  • MozillaFirefox SLE Module for Desktop Applications 15 SP3, SLE Module for Desktop Applications 15 SP2 91.4.0-152.9.1
  • MozillaFirefox SLES 11 SP4-LTSS 91.4.0-78.154.1

suse advisories SUSE-SU-2021:4150-1SUSE-SU-2021:4000-1SUSE-SU-2021:3995-1SUSE-SU-2021:14859-1SUSE-SU-2021:3993-1

  • firefox-esr openSUSE Tumbleweed 128.5.1-1.1
  • MozillaFirefox openSUSE Tumbleweed 95.0-1.1
  • MozillaThunderbird openSUSE Tumbleweed 91.4.0-1.1
  • MozillaThunderbird openSUSE Leap 15.2 91.4.0-lp152.2.52.1
  • MozillaThunderbird openSUSE Leap 15.3 91.4.0-8.45.2
  • MozillaFirefox openSUSE Leap 15.2 91.4.0-lp152.2.74.1
  • MozillaFirefox openSUSE Leap 15.3 91.4.0-152.9.1

opensuse advisories openSUSE-SU-2024:14572-1openSUSE-SU-2024:11669-1openSUSE-SU-2024:11670-1openSUSE-SU-2021:1635-1openSUSE-SU-2021:4150-1openSUSE-SU-2021:1575-1

  • thunderbird AlmaLinux 8 91.4.0-2.el8_5.alma
  • thunderbird AlmaLinux 8 91.4.0-2.el8_5.alma.plus
  • firefox AlmaLinux 8 91.4.0-1.el8_5.alma

almalinux advisories ALSA-2021:5045ALSA-2021:5013

  • 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

  • firefox Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 95.0-r0
  • librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 95.0-r0
  • firefox-esr Alpine 3.23, Alpine 3.22, Alpine 3.21 +6 more 91.4.0-r0
  • thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 91.4.0-r0

alpine advisories ALPINE-CVE-2021-43538

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

  • firefox Arch Linux 95.0-1
  • thunderbird Arch Linux 91.4.0-1

arch linux advisories AVG-2606AVG-2608

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-2021-43538 updated cvss 7.5

Mozilla: Missing fullscreen and pointer lock notification when requesting both

  • 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

Exploit prediction

1.20% chance of exploitation in the next 30 days, higher than 68% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 4.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
NVD 2.0 4.3 AV:N/AC:M/Au:N/C:N/I:P/A:N

Affected products

  • Mozilla Thunderbird
  • Mozilla Firefox ESR
  • Mozilla Firefox

Classification

Weakness
  • CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization

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.