The developer page about:memory has a Measure function for exploring what object types the browser has allocated and their sizes.

published 2021-02-26 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-10896

cvss: 6.5; 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

The developer page about:memory has a Measure function for exploring what object types the browser has allocated and their sizes. When this function was invoked we incorrectly called the sizeof function, instead of using the API method that checks for invalid pointers. This vulnerability affects Firefox < 86.

Patch / fix

fixed in 1 distros 4

mozilla firefox 86.0

ubuntu opensuse alpine arch linux

firefox Ubuntu 16.04 LTS 86.0+build3-0ubuntu0.16.04.1
firefox Ubuntu 18.04 LTS 86.0+build3-0ubuntu0.18.04.1
firefox Ubuntu 20.04 LTS 86.0+build3-0ubuntu0.20.04.1

ubuntu advisories UBUNTU-CVE-2021-23975

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 87.0-r0
librewolf Alpine 3.24, Alpine 3.23, Alpine 3.22 +1 more 87.0-r0

alpine advisories ALPINE-CVE-2021-23975

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 86.0-1

arch linux advisories AVG-1599

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	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

Classification

Weakness

CWE-862 Missing Authorization

References

security.gentoo.org /glsa/202104-10
vendor-advisory, third-party-advisory
mozilla.org /security/advisories/mfsa2021-07/
release-notes, vendor-advisory
bugzilla.mozilla.org /show_bug.cgi?id=1685145
issue-tracking, permissions-required, vendor-advisory

Sources

CVE List CVE-2021-23975
NVD CVE-2021-23975

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