CVE-2021-43535

Name: CVE-2021-43535
Creator: NVD / NIST
Published: 2021-12-08T22:15:09.517+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 8.8/10EPSS 1.11%

Last modified Jun 17, 2026

CVE-2021-43535 is a high-severity vulnerability rated 8.8/10 on the CVSS scale. A use-after-free could have occured when an HTTP2 session object was released on a different thread, leading to memory corruption and a potentially exploitable crash. This vulnerability affects Firefox < 93, Thunderbird < 91.3, and Firefox ESR < 91.3.. EPSS estimates a 1.11% chance of exploitation in the next 30 days.

Description

A use-after-free could have occured when an HTTP2 session object was released on a different thread, leading to memory corruption and a potentially exploitable crash. This vulnerability affects Firefox < 93, Thunderbird < 91.3, and Firefox ESR < 91.3.

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

1.11%

61.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-416

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 93.0
Mozilla	Firefox Esr	< 91.3.0
Mozilla	Thunderbird	< 91.3.0
Debian	Debian Linux	9.0
Debian	Debian Linux	10.0
Debian	Debian Linux	11.0

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1667102Issue Tracking, Permissions Required, Vendor Advisory
https://lists.debian.org/debian-lts-announce/2021/12/msg00030.htmlMailing List, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/01/msg00001.htmlMailing List, Third Party Advisory
https://www.debian.org/security/2021/dsa-5026Third Party Advisory
https://www.debian.org/security/2022/dsa-5034Issue Tracking, Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-43/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-49/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-50/Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1667102Issue Tracking, Permissions Required, Vendor Advisory
https://lists.debian.org/debian-lts-announce/2021/12/msg00030.htmlMailing List, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/01/msg00001.htmlMailing List, Third Party Advisory
https://www.debian.org/security/2021/dsa-5026Third Party Advisory
https://www.debian.org/security/2022/dsa-5034Issue Tracking, Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-43/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-49/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-50/Vendor Advisory

Timeline

Published: Dec 8, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-43535?

How severe is CVE-2021-43535?

CVE-2021-43535 has a CVSS score of 8.8/10 (HIGH severity). The EPSS model estimates a 1.11% probability of exploitation in the next 30 days.

How do I fix CVE-2021-43535?

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-2021-43535?

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

Scan your code now

Source: NVD / NIST