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Fencer Vulns
medium 6.5

CVE-2021-23984 json

A malicious extension could have opened a popup window lacking an address bar.

published 2021-03-31 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-10904

cvss
6.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.1%
EPSS 1.1% chance of exploitation in 30 days, percentile 66%
public exploits
0
none known

Summary

A malicious extension could have opened a popup window lacking an address bar. The title of the popup lacking an address bar should not be fully controllable, but in this situation was. This could have been used to spoof a website and attempt to trick the user into providing credentials. This vulnerability affects Firefox ESR < 78.9, Firefox < 87, and Thunderbird < 78.9.

Patch / fix

  • Mozilla Firefox ESR 78.9
  • Mozilla Firefox 87
  • Mozilla Thunderbird 78.9
  • firefox-esr Debian 14, Debian 13, Debian 12 78.9.0esr-1
  • thunderbird Debian 14, Debian 13, Debian 12 1:78.9.0-1

debian advisories DEBIAN-CVE-2021-23984

  • firefox Ubuntu 16.04 LTS 87.0+build3-0ubuntu0.16.04.2
  • firefox Ubuntu 18.04 LTS 87.0+build3-0ubuntu0.18.04.2
  • firefox Ubuntu 20.04 LTS 87.0+build3-0ubuntu0.20.04.2
  • thunderbird Ubuntu 18.04 LTS 1:78.11.0+build1-0ubuntu0.18.04.2
  • thunderbird Ubuntu 20.04 LTS 1:78.11.0+build1-0ubuntu0.20.04.2

ubuntu advisories UBUNTU-CVE-2021-23984

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 78.9.0-3.el7_9
  • firefox RHEL 7 78.9.0-1.el7_9
  • thunderbird RHEL 8 78.9.0-3.el8_3
  • firefox RHEL 8 78.9.0-1.el8_3
  • firefox RHEL 8.1 EUS 78.9.0-1.el8_1
  • thunderbird RHEL 8.2 EUS 78.9.0-3.el8_2
  • thunderbird RHEL 8.1 EUS 78.9.0-3.el8_1
  • firefox RHEL 8.2 EUS 78.9.0-1.el8_2

red hat advisories RHSA-2021:0996RHSA-2021:0991RHSA-2021:0994RHSA-2021:0992RHSA-2021:0995RHSA-2021:0993

  • MozillaThunderbird SLE Workstation Extension 15 SP2 78.9.1-8.20.1
  • MozillaFirefox SLES for SAP 15 SP1, SLES for SAP 15, SLES 15-LTSS +10 more 78.9.0-3.136.1
  • mozilla-nspr SLES for SAP 15 SP1, SLES for SAP 15, SLES 15-LTSS +12 more 4.25.1-3.17.1
  • MozillaFirefox-branding-SLE SLES for SAP 15 SP1, SLES for SAP 15, SLES 15-LTSS +10 more 78-4.16.1
  • MozillaFirefox SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +14 more 78.9.0-112.54.1
  • MozillaFirefox SLE Module for Desktop Applications 15 SP2 78.9.0-8.35.1
  • MozillaFirefox SLES 11 SP4-LTSS 78.9.0-78.123.1

suse advisories SUSE-SU-2021:1167-1SUSE-SU-2021:1007-1SUSE-SU-2021:14684-1SUSE-SU-2021:0999-1SUSE-SU-2021:0966-1

  • firefox-esr openSUSE Tumbleweed 128.5.1-1.1
  • MozillaFirefox openSUSE Tumbleweed 92.0-1.2
  • MozillaThunderbird openSUSE Tumbleweed 91.1.1-1.1
  • MozillaThunderbird openSUSE Leap 15.2 78.9.1-lp152.2.38.1
  • MozillaFirefox openSUSE Leap 15.2 78.9.0-lp152.2.52.1

opensuse advisories openSUSE-SU-2024:14572-1openSUSE-SU-2024:10600-1openSUSE-SU-2024:10601-1openSUSE-SU-2021:0580-1openSUSE-SU-2021:0487-1

  • firefox Alpine 3.23, Alpine 3.22, Alpine 3.21 +8 more 87.0-r0
  • librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 87.0-r0
  • firefox-esr Alpine 3.23, Alpine 3.22, Alpine 3.21 +9 more 78.9.0-r0
  • thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 78.9.0-r0

alpine advisories ALPINE-CVE-2021-23984

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 87.0-1
  • thunderbird Arch Linux 78.9.0-1

arch linux advisories AVG-1728AVG-1729

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-2021-23984 updated cvss 6.1

Mozilla: Malicious extensions could have spoofed popup information

  • Red Hat Enterprise Linux 6 firefox, thunderbird
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux AppStream (v. 8) firefox 78.9.0-1.el8_3, thunderbird 78.9.0-3.el8_3
    fixed: vendor fix RHSA-2021:0990
  • Red Hat Enterprise Linux AppStream EUS (v. 8.1) firefox 78.9.0-1.el8_1, thunderbird 78.9.0-3.el8_1
    fixed: vendor fix RHSA-2021:0991
  • Red Hat Enterprise Linux AppStream EUS (v. 8.2) firefox 78.9.0-1.el8_2, thunderbird 78.9.0-3.el8_2
    fixed: vendor fix RHSA-2021:0989
  • Red Hat Enterprise Linux Client (v. 7) firefox 78.9.0-1.el7_9, thunderbird 78.9.0-3.el7_9 Red Hat Enterprise Linux Client Optional (v. 7) firefox 78.9.0-1.el7_9 Red Hat Enterprise Linux Server (v. 7) firefox 78.9.0-1.el7_9 Red Hat Enterprise Linux Server Optional (v. 7) firefox 78.9.0-1.el7_9, thunderbird 78.9.0-3.el7_9 Red Hat Enterprise Linux Workstation (v. 7) firefox 78.9.0-1.el7_9, thunderbird 78.9.0-3.el7_9 Red Hat Enterprise Linux Workstation Optional (v. 7) firefox 78.9.0-1.el7_9
    fixed: vendor fix RHSA-2021:0992

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

1.11% chance of exploitation in the next 30 days, higher than 66% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
NVD 2.0 4.3 AV:N/AC:M/Au:N/C:N/I:P/A:N

Affected products

  • Mozilla Firefox ESR
  • Mozilla Firefox
  • Mozilla Thunderbird

Classification

Weakness
  • CWE-290 Authentication Bypass by Spoofing

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.