CVE-2021-23960

Name: CVE-2021-23960
Creator: NVD / NIST
Published: 2021-02-26T03:15:13.917+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 8.8/10EPSS 1.21%

Last modified Jun 17, 2026

CVE-2021-23960 is a high-severity vulnerability rated 8.8/10 on the CVSS scale. Performing garbage collection on re-declared JavaScript variables resulted in a user-after-poison, and a potentially exploitable crash. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.. EPSS estimates a 1.21% chance of exploitation in the next 30 days.

Description

Performing garbage collection on re-declared JavaScript variables resulted in a user-after-poison, and a potentially exploitable crash. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.

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

1.21%

64.4th percentile

Probability of exploitation in the next 30 days. Learn more

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 85.0
Mozilla	Firefox Esr	< 78.7
Mozilla	Thunderbird	< 78.7

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1675755Issue Tracking, Permissions Required, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-03/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-04/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-05/Release Notes, Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1675755Issue Tracking, Permissions Required, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-03/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-04/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-05/Release Notes, Vendor Advisory

Timeline

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

Frequently Asked Questions

What is CVE-2021-23960?

How severe is CVE-2021-23960?

CVE-2021-23960 has a CVSS score of 8.8/10 (HIGH severity). The EPSS model estimates a 1.21% probability of exploitation in the next 30 days.

How do I fix CVE-2021-23960?

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-2021-23960?

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

Scan your code now

Source: NVD / NIST