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Fencer Vulns
high 8.8

CVE-2021-43535 json

A use-after-free could have occured when an HTTP2 session object was released on a different thread, leading to memory corruption and a pot…

published 2021-12-08 4 y ago modified 2026-06-17 4 mo ago also EUVD-2021-30464

cvss
8.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.1%
EPSS 1.1% chance of exploitation in 30 days, percentile 66%
public exploits
0
none known

Summary

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.

Patch / fix

  • Mozilla Firefox 93
  • Mozilla Thunderbird 91.3
  • Mozilla Firefox ESR 91.3
  • firefox-esr Debian 14, Debian 13, Debian 12 91.3.0esr-1
  • thunderbird Debian 14, Debian 13, Debian 12 1:91.3.0-1

debian advisories DEBIAN-CVE-2021-43535

  • thunderbird Ubuntu 18.04 LTS 1:91.5.0+build1-0ubuntu0.18.04.1
  • thunderbird Ubuntu 20.04 LTS 1:91.5.0+build1-0ubuntu0.20.04.1

ubuntu advisories UBUNTU-CVE-2021-43535

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

  • thunderbird RHEL 7 91.3.0-2.el7_9
  • thunderbird RHEL 8 91.3.0-2.el8_4
  • firefox RHEL 8 91.3.0-1.el8_4
  • firefox RHEL 7 91.3.0-1.el7_9
  • thunderbird RHEL 8.1 EUS 91.3.0-2.el8_1
  • thunderbird RHEL 8.2 EUS 91.3.0-2.el8_2
  • firefox RHEL 8.2 EUS 91.3.0-1.el8_2
  • firefox RHEL 8.1 EUS 91.3.0-1.el8_1

red hat advisories RHSA-2021:4133RHSA-2021:4132RHSA-2021:4134RHSA-2021:4605RHSA-2021:4130RHSA-2021:4607

  • thunderbird Rocky Linux 8 91.3.0-2.el8_4
  • thunderbird Rocky Linux 8 91.3.0-2.el8
  • firefox Rocky Linux 8 91.3.0-1.el8

rocky linux advisories RLSA-2021:4130RLSA-2021:4123

  • thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 91.3.2-r0

alpine advisories ALPINE-CVE-2021-43535

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-43535 updated cvss 8.8

Mozilla: Use-after-free in HTTP2 Session object

  • Red Hat Enterprise Linux 6 firefox, thunderbird
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux AppStream (v. 8) firefox 91.3.0-1.el8_4, thunderbird 91.3.0-2.el8_4
    fixed: vendor fix RHSA-2021:4123
  • Red Hat Enterprise Linux AppStream EUS (v. 8.1) firefox 91.3.0-1.el8_1, thunderbird 91.3.0-2.el8_1
    fixed: vendor fix RHSA-2021:4607
  • Red Hat Enterprise Linux AppStream EUS (v. 8.2) firefox 91.3.0-1.el8_2, thunderbird 91.3.0-2.el8_2
    fixed: vendor fix RHSA-2021:4605
  • Red Hat Enterprise Linux Client (v. 7) firefox 91.3.0-1.el7_9, thunderbird 91.3.0-2.el7_9 Red Hat Enterprise Linux Client Optional (v. 7) firefox 91.3.0-1.el7_9 Red Hat Enterprise Linux Server (v. 7) firefox 91.3.0-1.el7_9 Red Hat Enterprise Linux Server Optional (v. 7) firefox 91.3.0-1.el7_9, thunderbird 91.3.0-2.el7_9 Red Hat Enterprise Linux Workstation (v. 7) firefox 91.3.0-1.el7_9, thunderbird 91.3.0-2.el7_9 Red Hat Enterprise Linux Workstation Optional (v. 7) firefox 91.3.0-1.el7_9
    fixed: vendor fix RHSA-2021:4116

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

1.15% chance of exploitation in the next 30 days, higher than 66% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 8.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
NVD 2.0 6.8 AV:N/AC:M/Au:N/C:P/I:P/A:P

Affected products

  • Mozilla Firefox
  • Mozilla Thunderbird
  • Mozilla Firefox ESR

Classification

Weakness

References

Sources

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.