# CVE-2020-6820: Mozilla Firefox And Thunderbird Use-After-Free

> A race condition in ReadableStream handling leads to a use-after-free, allowing attackers to execute arbitrary code via targeted attacks.

- **Severity:** High 8.1 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Yes, since 3 Nov 2021 (CISA KEV)
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 24 Apr 2020
- **Updated:** 21 Oct 2025
- **Also known as:** DEBIAN-CVE-2020-6820, GHSA-f77r-rqc9-53hh, UBUNTU-CVE-2020-6820

## What is CVE-2020-6820?

CVE-2020-6820 is a high-severity use after free vulnerability in Mozilla Firefox, published on 24 Apr 2020. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free. We are aware of targeted attacks in the wild abusing this flaw. This vulnerability affects Thunderbird < 68.7.0, Firefox < 74.0.1, and Firefox ESR < 68.6.1.

Weakness: CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization)

## Which versions are affected by CVE-2020-6820?

It affects 2 products.


Products: Mozilla Firefox, Mozilla Thunderbird.

## How do I fix CVE-2020-6820?

Upgrade to a fixed version. US federal agencies must fix it by 3 May 2022.

## Is CVE-2020-6820 being exploited?

Yes. Attackers have exploited CVE-2020-6820 in the wild since at least 3 Nov 2021.

## 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-2020-6820 and fix it if it is affected.

What it is: CVE-2020-6820 is a high-severity use after free vulnerability in Mozilla Firefox, published on 24 Apr 2020. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: Yes. Attackers have exploited CVE-2020-6820 in the wild since at least 3 Nov 2021.

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-2020-6820/ (Markdown: https://insights.sbomify.com/CVE-2020-6820/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-6820
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-6820
- OSV: https://osv.dev/vulnerability/CVE-2020-6820

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From sbomify Insights: https://insights.sbomify.com/CVE-2020-6820/
Summaries are generated and may contain mistakes; the official description above is the record.