CVE-2021-43538 Race condition in Firefox

What is CVE-2021-43538?

CVE-2021-43538 is a medium-severity vulnerability in Debian Debian Linux, published on 8 Dec 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official description

By misusing a race in our notification code, an attacker could have forcefully hidden the notification for pages that had received full screen and pointer lock access, which could have been used for spoofing attacks. This vulnerability affects Thunderbird < 91.4.0, Firefox ESR < 91.4.0, and Firefox < 95.

Weakness: CWE-362

Which versions are affected by CVE-2021-43538?

It affects 4 products.

Products include Debian Debian Linux, Mozilla Firefox, Mozilla Firefox Esr, Mozilla Thunderbird.

How do I fix CVE-2021-43538?

Upgrade to a fixed version.

Is CVE-2021-43538 being exploited?

There is no known exploitation of CVE-2021-43538 in the wild and no public exploit code.

Other vulnerabilities in Debian Debian Linux

• CVE-2015-3306 ProFTPD Improper Access Control Exploited Critical 10.0
• CVE-2025-49113 RoundCube Webmail Deserialization of Untrusted Data Exploited Critical 9.9
• CVE-2026-24061 GNU InetUtils Argument Injection Exploited Critical 9.8
• CVE-2025-39682 Linux Kernel Improper Check for Unusual or Exceptional Conditions Exploited Critical 9.8
• CVE-2024-9680 Mozilla Firefox Use-After-Free Exploited Critical 9.8

All Debian Debian Linux vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as DEBIAN-CVE-2021-43538, GHSA-9mvw-9c9q-7cmm, UBUNTU-CVE-2021-43538.