Mozilla developers reported memory safety bugs present in Firefox 89.

published 2021-08-05 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-16436

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 89. 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 < 90.

Patch / fix

fixed in 1 distros 4

Mozilla Firefox 90

ubuntu opensuse alpine arch linux

firefox Ubuntu 18.04 LTS 90.0+build1-0ubuntu0.18.04.1
firefox Ubuntu 20.04 LTS 90.0+build1-0ubuntu0.20.04.1

ubuntu advisories UBUNTU-CVE-2021-29977

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 +6 more 90.0-r0
librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 90.0-r0

alpine advisories ALPINE-CVE-2021-29977

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 90.0-1

arch linux advisories AVG-2148

Exploit prediction

1.01% 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/202202-03
vendor-advisory, third-party-advisory
mozilla.org /security/advisories/mfsa2021-28/
vendor-advisory
bugzilla.mozilla.org /buglist.cgi?bug_id=1665836%2C1686138%2C1704316%2C1706314%2C1709931%2C1712084%2C1712357%2C1714066
issue-tracking, vendor-advisory

Sources

CVE List CVE-2021-29977
NVD CVE-2021-29977

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