CVE-2021-23978

Name: CVE-2021-23978
Creator: NVD / NIST
Published: 2021-02-26T16:15:12.673+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 8.8/10EPSS 1.54%

Last modified Jun 17, 2026

CVE-2021-23978 is a high-severity vulnerability rated 8.8/10 on the CVSS scale. Mozilla developers reported memory safety bugs present in Firefox 85 and Firefox ESR 78.7. 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. EPSS estimates a 1.54% chance of exploitation in the next 30 days.

Description

Mozilla developers reported memory safety bugs present in Firefox 85 and Firefox ESR 78.7. 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 < 86, Thunderbird < 78.8, and Firefox ESR < 78.8.

Metrics

CVSS 3.1

8.8/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

EPSS Probability

1.54%

71.8th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-787

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 86.0
Mozilla	Firefox Esr	< 78.8
Mozilla	Thunderbird	< 78.8
Debian	Debian Linux	9.0
Debian	Debian Linux	10.0

References

https://bugzilla.mozilla.org/buglist.cgi?bug_id=786797%2C1682928%2C1687391%2C1687597Broken Link, Issue Tracking, Vendor Advisory
https://lists.debian.org/debian-lts-announce/2021/03/msg00000.htmlMailing List, Third Party Advisory
https://security.gentoo.org/glsa/202104-09Third Party Advisory
https://security.gentoo.org/glsa/202104-10Third Party Advisory
https://www.debian.org/security/2021/dsa-4866Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-07/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-08/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-09/Release Notes, Vendor Advisory
https://bugzilla.mozilla.org/buglist.cgi?bug_id=786797%2C1682928%2C1687391%2C1687597Broken Link, Issue Tracking, Vendor Advisory
https://lists.debian.org/debian-lts-announce/2021/03/msg00000.htmlMailing List, Third Party Advisory
https://security.gentoo.org/glsa/202104-09Third Party Advisory
https://security.gentoo.org/glsa/202104-10Third Party Advisory
https://www.debian.org/security/2021/dsa-4866Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-07/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-08/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-09/Release Notes, Vendor Advisory

Timeline

Published: Feb 26, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-23978?

How severe is CVE-2021-23978?

CVE-2021-23978 has a CVSS score of 8.8/10 (HIGH severity). The EPSS model estimates a 1.54% probability of exploitation in the next 30 days.

How do I fix CVE-2021-23978?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-23978?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST