# CVE-2021-4140: Security feature bypass in Firefox

> Attackers can bypass iframe sandbox restrictions using specific XSLT markup in Firefox and Thunderbird, requiring users to upgrade to patched versions.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 22 Dec 2022
- **Updated:** 16 Apr 2025
- **Also known as:** DEBIAN-CVE-2021-4140, GHSA-h5mr-xp97-c4p5, UBUNTU-CVE-2021-4140

## What is CVE-2021-4140?

CVE-2021-4140 is a critical vulnerability in Mozilla Firefox, published on 22 Dec 2022. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

It was possible to construct specific XSLT markup that would be able to bypass an iframe sandbox. This vulnerability affects Firefox ESR < 91.5, Firefox < 96, and Thunderbird < 91.5.

Weakness: CWE-91 (XML Injection)

## Which versions are affected by CVE-2021-4140?

It affects 3 products.


Products: Mozilla Firefox, Mozilla Firefox Esr, Mozilla Thunderbird.

## How do I fix CVE-2021-4140?

Upgrade to a fixed version.

## Is CVE-2021-4140 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-4140 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-2021-4140 and fix it if it is affected.

What it is: CVE-2021-4140 is a critical vulnerability in Mozilla Firefox, published on 22 Dec 2022. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-4140 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-2021-4140/ (Markdown: https://insights.sbomify.com/CVE-2021-4140/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-4140
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-4140
- OSV: https://osv.dev/vulnerability/CVE-2021-4140

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-4140/
Summaries are generated and may contain mistakes; the official description above is the record.