Performing garbage collection on re-declared JavaScript variables resulted in a user-after-poison, and a potentially exploitable crash.

published 2021-02-26 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-10881

cvss: 8.8; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 1.3%; EPSS 1.3% chance of exploitation in 30 days, percentile 70%
public exploits: 0; none known

Performing garbage collection on re-declared JavaScript variables resulted in a user-after-poison, and a potentially exploitable crash. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.

Patch / fix

fixed in 3 distros 7

mozilla firefox 85.0
mozilla firefox_esr 78.7
mozilla thunderbird 78.7

debian ubuntu red hat suse opensuse alpine arch linux

firefox-esr Debian 14, Debian 13, Debian 12 78.7.0esr-1
thunderbird Debian 14, Debian 13, Debian 12 1:78.7.0-1

debian advisories DEBIAN-CVE-2021-23960

firefox Ubuntu 16.04 LTS 85.0+build1-0ubuntu0.16.04.1
firefox Ubuntu 18.04 LTS 85.0+build1-0ubuntu0.18.04.1
firefox Ubuntu 20.04 LTS 85.0+build1-0ubuntu0.20.04.1
thunderbird Ubuntu 18.04 LTS 1:78.8.1+build1-0ubuntu0.18.04.1
thunderbird Ubuntu 20.04 LTS 1:78.7.1+build1-0ubuntu0.20.04.1

ubuntu advisories UBUNTU-CVE-2021-23960

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 78.7.0-1.el7_9
thunderbird RHEL 8 78.7.0-1.el8_3
firefox RHEL 7 78.7.0-2.el7_9
firefox RHEL 8 78.7.0-2.el8_3
thunderbird RHEL 8.1 EUS 78.7.0-1.el8_1
thunderbird RHEL 8.2 EUS 78.7.0-1.el8_2
firefox RHEL 8.2 EUS 78.7.0-2.el8_2
firefox RHEL 8.1 EUS 78.7.0-2.el8_1

red hat advisories RHSA-2021:0397 RHSA-2021:0297 RHSA-2021:0298 RHSA-2021:0299 RHSA-2021:0290 RHSA-2021:0289

MozillaFirefox SLE Module for Desktop Applications 15 SP2 78.7.0-8.26.2
MozillaThunderbird SLE Workstation Extension 15 SP2 78.7.0-8.9.1
MozillaFirefox SLES for SAP 15 SP1, SLES 15 SP1-LTSS, SLES 15 SP1-BCL +7 more 78.7.0-3.128.2
MozillaThunderbird SLE Workstation Extension 15 SP1 78.7.0-3.119.1
MozillaFirefox SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +15 more 78.7.0-112.45.1
MozillaFirefox SLES 11 SP4-LTSS 78.7.0-78.114.1

suse advisories SUSE-SU-2021:0259-1 SUSE-SU-2021:0257-1 SUSE-SU-2021:0246-1 SUSE-SU-2021:0245-1 SUSE-SU-2021:14609-1 SUSE-SU-2021:0241-1

firefox-esr openSUSE Tumbleweed 128.5.1-1.1
MozillaFirefox openSUSE Tumbleweed 92.0-1.2
MozillaThunderbird openSUSE Tumbleweed 91.1.1-1.1
MozillaFirefox openSUSE Leap 15.2 78.7.0-lp152.2.43.1
MozillaFirefox openSUSE Leap 15.1 78.7.0-lp151.2.88.2
MozillaThunderbird openSUSE Leap 15.2 78.7.0-lp152.2.29.1
MozillaThunderbird openSUSE Leap 15.1 78.7.0-lp151.2.69.1

opensuse advisories openSUSE-SU-2024:14572-1 openSUSE-SU-2024:10600-1 openSUSE-SU-2024:10601-1 openSUSE-SU-2021:0223-1 openSUSE-SU-2021:0222-1 openSUSE-SU-2021:0209-1

firefox Alpine 3.23, Alpine 3.22, Alpine 3.21 +8 more 85.0-r0
mozjs78 Alpine 3.16, Alpine 3.15, Alpine 3.14 +1 more 78.7.0-r0
librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 85.0-r0
firefox-esr Alpine 3.23, Alpine 3.22, Alpine 3.21 +9 more 78.7.0-r0
thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 78.7.0-r0

alpine advisories ALPINE-CVE-2021-23960

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 85.0-1
thunderbird Arch Linux 78.7.0-1

arch linux advisories AVG-1492 AVG-1496

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-2021-23960 updated 2025-11-21 cvss 8.8

Mozilla: Use-after-poison for incorrectly redeclared JavaScript variables during GC

Red Hat Enterprise Linux 6 firefox, thunderbird
affected: no fix planned “Out of support scope”
Red Hat Enterprise Linux AppStream (v. 8) firefox 78.7.0-2.el8_3, thunderbird 78.7.0-1.el8_3
fixed: vendor fix RHSA-2021:0288
Red Hat Enterprise Linux AppStream EUS (v. 8.1) firefox 78.7.0-2.el8_1, thunderbird 78.7.0-1.el8_1
fixed: vendor fix RHSA-2021:0285
Red Hat Enterprise Linux AppStream EUS (v. 8.2) firefox 78.7.0-2.el8_2, thunderbird 78.7.0-1.el8_2
fixed: vendor fix RHSA-2021:0289
Red Hat Enterprise Linux Client (v. 7) firefox 78.7.0-2.el7_9, thunderbird 78.7.0-1.el7_9 Red Hat Enterprise Linux Client Optional (v. 7) firefox 78.7.0-2.el7_9 Red Hat Enterprise Linux Server (v. 7) firefox 78.7.0-2.el7_9 Red Hat Enterprise Linux Server Optional (v. 7) firefox 78.7.0-2.el7_9, thunderbird 78.7.0-1.el7_9 Red Hat Enterprise Linux Workstation (v. 7) firefox 78.7.0-2.el7_9, thunderbird 78.7.0-1.el7_9 Red Hat Enterprise Linux Workstation Optional (v. 7) firefox 78.7.0-2.el7_9
fixed: vendor fix RHSA-2021:0290

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

1.29% chance of exploitation in the next 30 days, higher than 70% 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

References

mozilla.org /security/advisories/mfsa2021-03/
release-notes, vendor-advisory
mozilla.org /security/advisories/mfsa2021-04/
release-notes, vendor-advisory
mozilla.org /security/advisories/mfsa2021-05/
release-notes, vendor-advisory
bugzilla.mozilla.org /show_bug.cgi?id=1675755
issue-tracking, permissions-required, vendor-advisory

Sources

CVE List CVE-2021-23960
NVD CVE-2021-23960

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.

Summary