# CVE-2021-43538: Race condition in Firefox

> Attackers can hide full-screen notifications via a race condition, enabling spoofing attacks in Firefox and Thunderbird.

- **Severity:** Medium 4.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 8 Dec 2021
- **Updated:** 4 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-43538, GHSA-9mvw-9c9q-7cmm, UBUNTU-CVE-2021-43538

## 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 (Concurrent Execution using Shared Resource with Improper Synchronization)

## Which versions are affected by CVE-2021-43538?

It affects 4 products.


Products: 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](https://insights.sbomify.com/CVE-2015-3306/): ProFTPD Improper Access Control (critical 10.0, exploited in the wild)
- [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild)
- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-43538 and fix it if it is affected.

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

Affected product: Debian Debian Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-43538/ (Markdown: https://insights.sbomify.com/CVE-2021-43538/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-43538
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-43538
- OSV: https://osv.dev/vulnerability/CVE-2021-43538

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-43538/
Summaries are generated and may contain mistakes; the official description above is the record.