---
title: "CVE-2021-43535\n    | Ubuntu"
description: Ubuntu is an open source software operating system that runs from the
  desktop, to the cloud, to all your internet connected things.
url: https://ubuntu.com/security/CVE-2021-43535?format=md
keywords: index, follow
---

# CVE-2021-43535

Publication date 8 December 2021

Last updated 25 August 2025

---

Ubuntu priority

**Medium**

[Why this priority?](https://ubuntu.com/security/cves/about#priority )

## Cvss 3 Severity Score

**8.8 · High**

[Score breakdown](https://ubuntu.com/security/CVE-2021-43535?format=md#impact-score)

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## 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.

[Read the notes from the security team](https://ubuntu.com/security/CVE-2021-43535?format=md#notes)

## Status

Show unmaintained releases

| Package | Ubuntu Release | Status |
| --- | --- | --- |
| firefox | 23.04 lunar | Not affected |
| 22.10 kinetic | Not affected |
| 22.04 LTS jammy | Not affected |
| 21.10 impish | Not affected |
| 21.04 hirsute | Not affected |
| 20.04 LTS focal | Not in release |
| 18.04 LTS bionic | Not affected |
| 16.04 LTS xenial | Ignored end of standard support, was needed |
| 14.04 LTS trusty | Not in release |
| thunderbird | 23.04 lunar | Not affected |
| 22.10 kinetic | Not affected |
| 22.04 LTS jammy | Not affected |
| 21.10 impish | Not affected |
| 21.04 hirsute | Ignored end of life |
| 20.04 LTS focal | Fixed 1:91.5.0+build1-0ubuntu0.20.04.1 |
| 18.04 LTS bionic | Fixed 1:91.5.0+build1-0ubuntu0.18.04.1 |
| 16.04 LTS xenial | Ignored end of standard support, was needed |
| 14.04 LTS trusty | Not in release |

---

* [How can I get the fixes?](https://ubuntu.com/security/cves/about#security)
* [What do statuses mean?](https://ubuntu.com/security/cves/about#statuses)

## Notes

---

### [tyhicks](https://launchpad.net/~tyhicks)

mozjs contains a copy of the SpiderMonkey JavaScript engine

## Severity score breakdown

CVSS version:
CVSS v3.0

**Base score**

8.8 · High

* Base metrics

  | Parameter | Value |
  | --- | --- |
  | Attack vector | Network |
  | Attack complexity | Low |
  | Privileges required | None |
  | User interaction | Required |
  | Scope | Unchanged |
  | Confidentiality impact | High |
  | Integrity impact | High |
  | Availability impact | High |
* Scores

  | Parameter | Value |
  | --- | --- |
  | Base score | 8.8 · High |
  | Exploitability score | - |
  | Impact score | - |

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

## References

* [MITRE](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-43535)
* [NVD](https://nvd.nist.gov/vuln/detail/CVE-2021-43535)
* [Launchpad](https://launchpad.net/bugs/cve/CVE-2021-43535)
* [Debian](https://security-tracker.debian.org/tracker/CVE-2021-43535)

### Related Ubuntu Security Notices (USN)

+ [USN-5248-1](https://usn.ubuntu.com/USN-5248-1)
+ Thunderbird vulnerabilities
+ 21 January 2022

### Other references

* <https://www.mozilla.org/security/advisories/mfsa2021-50/>
* <https://www.mozilla.org/security/advisories/mfsa2021-43/>
* <https://www.mozilla.org/security/advisories/mfsa2021-49/>
* <https://bugzilla.mozilla.org/show_bug.cgi?id=1667102>
* <https://www.cve.org/CVERecord?id=CVE-2021-43535>