Mozilla developers reported memory safety bugs present in Firefox 88.

published 2021-06-24 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-16425

cvss: 8.8; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 1.0%; EPSS 1.0% chance of exploitation in 30 days, percentile 63%
public exploits: 0; none known

Mozilla developers reported memory safety bugs present in Firefox 88. 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 < 89.

Patch / fix

fixed in 1 distros 4

Mozilla Firefox 89

ubuntu opensuse alpine arch linux

firefox Ubuntu 18.04 LTS 89.0+build2-0ubuntu0.18.04.2
firefox Ubuntu 20.04 LTS 89.0+build2-0ubuntu0.20.04.2

ubuntu advisories UBUNTU-CVE-2021-29966

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

firefox-esr openSUSE Tumbleweed 128.5.1-1.1
MozillaFirefox openSUSE Tumbleweed 92.0-1.2

opensuse advisories openSUSE-SU-2024:14572-1 openSUSE-SU-2024:10600-1

firefox Alpine 3.23, Alpine 3.22, Alpine 3.21 +8 more 89.0-r0
librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 89.0-r0

alpine advisories ALPINE-CVE-2021-29966

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 89.0-1

arch linux advisories AVG-2018

Exploit prediction

1.02% chance of exploitation in the next 30 days, higher than 63% 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

Classification

Weakness

CWE-787 Out-of-bounds Write

References

security.gentoo.org /glsa/202107-09
vendor-advisory, third-party-advisory
mozilla.org /security/advisories/mfsa2021-23/
release-notes, vendor-advisory
bugzilla.mozilla.org /buglist.cgi?bug_id=1660307%2C1686154%2C1702948%2C1708124
broken-link, issue-tracking

Sources

CVE List CVE-2021-29966
NVD CVE-2021-29966

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