Mozilla developers and community members reported memory safety bugs present in Firefox 87.

published 2021-06-24 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-16406

cvss: 8.8; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 0.9%; EPSS 0.9% chance of exploitation in 30 days, percentile 58%
public exploits: 0; none known

Mozilla developers and community members reported memory safety bugs present in Firefox 87. 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 < 88.

Patch / fix

fixed in 1 distros 4

Mozilla Firefox 88

ubuntu opensuse alpine arch linux

firefox Ubuntu 16.04 LTS 88.0+build2-0ubuntu0.16.04.1
firefox Ubuntu 18.04 LTS 88.0+build2-0ubuntu0.18.04.2
firefox Ubuntu 20.04 LTS 88.0+build2-0ubuntu0.20.04.1

ubuntu advisories UBUNTU-CVE-2021-29947

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 88.0-r0
librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 88.0-r0

alpine advisories ALPINE-CVE-2021-29947

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 88.0-1

arch linux advisories AVG-1834

Exploit prediction

0.87% chance of exploitation in the next 30 days, higher than 58% 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

mozilla.org /security/advisories/mfsa2021-16/
release-notes, vendor-advisory
bugzilla.mozilla.org /buglist.cgi?bug_id=1651449%2C1674142%2C1693476%2C1696886%2C1700091
broken-link, vendor-advisory

Sources

CVE List CVE-2021-29947
NVD CVE-2021-29947

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