CVE-2021-23995

Name: CVE-2021-23995
Creator: NVD / NIST
Published: 2021-06-24T14:15:09.357+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 8.8/10EPSS 1.21%

Last modified Jun 17, 2026

CVE-2021-23995 is a high-severity vulnerability rated 8.8/10 on the CVSS scale. When Responsive Design Mode was enabled, it used references to objects that were previously freed. We presume that with enough effort this could have been exploited to run arbitrary code. EPSS estimates a 1.21% chance of exploitation in the next 30 days.

Description

When Responsive Design Mode was enabled, it used references to objects that were previously freed. We presume that with enough effort this could have been exploited to run arbitrary code. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.

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%

64.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-672

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 88.0
Mozilla	Firefox Esr	< 78.10
Mozilla	Thunderbird	< 78.10

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1699835Issue Tracking, Permissions Required, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-14/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-15/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-16/Release Notes, Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1699835Issue Tracking, Permissions Required, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-14/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-15/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-16/Release Notes, Vendor Advisory

Timeline

Published: Jun 24, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-23995?

How severe is CVE-2021-23995?

CVE-2021-23995 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-2021-23995?

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-2021-23995?

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

Scan your code now

Source: NVD / NIST