# CVE-2021-4128: Use after free in Firefox

> A use-after-free vulnerability in Firefox on macOS allows attackers to cause memory corruption and potentially execute code via fullscreen transitions.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H`)
- **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:** No
- **Published:** 22 Dec 2022
- **Updated:** 16 Apr 2025
- **Also known as:** GHSA-qx74-qh3q-h8rj

## What is CVE-2021-4128?

CVE-2021-4128 is a medium-severity use after free vulnerability in Mozilla Firefox, published on 22 Dec 2022. 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

When transitioning in and out of fullscreen mode, a graphics object was not correctly protected; resulting in memory corruption and a potentially exploitable crash.<br>*This bug only affects Firefox on MacOS. Other operating systems are unaffected.*. This vulnerability affects Firefox < 95.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2021-4128?

It affects 1 product.


Products: Mozilla Firefox.

## How do I fix CVE-2021-4128?

No fix has been published for CVE-2021-4128 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-4128 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-4128 exists.

## Other vulnerabilities in Mozilla Firefox

- [CVE-2019-11708](https://insights.sbomify.com/CVE-2019-11708/): Mozilla Firefox and Thunderbird Sandbox Escape (critical 10.0, exploited in the wild)
- [CVE-2010-3765](https://insights.sbomify.com/CVE-2010-3765/): Mozilla Multiple Products Remote Code Execution (critical 9.8, exploited in the wild)
- [CVE-2022-26486](https://insights.sbomify.com/CVE-2022-26486/): Mozilla Firefox Use-After-Free (critical 9.6, exploited in the wild)
- [CVE-2022-26485](https://insights.sbomify.com/CVE-2022-26485/): Mozilla Firefox Use-After-Free (high 8.8, exploited in the wild)
- [CVE-2019-11707](https://insights.sbomify.com/CVE-2019-11707/): Mozilla Firefox and Thunderbird Type Confusion (high 8.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-4128 and fix it if it is affected.

What it is: CVE-2021-4128 is a medium-severity use after free vulnerability in Mozilla Firefox, published on 22 Dec 2022. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-4128 exists.

Affected product: Mozilla Firefox. 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-2021-4128/ (Markdown: https://insights.sbomify.com/CVE-2021-4128/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-4128
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-4128
- OSV: https://osv.dev/vulnerability/CVE-2021-4128

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-4128/
Summaries are generated and may contain mistakes; the official description above is the record.