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

Source: https://junglewise.ai/threats/cve-2020-6820-mozilla-firefox-and-thunderbird-use-after-free-vulnerability  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2021-11-03. Updated: 2026-05-10  
License: CC BY 4.0 with a link back

CVE-2020-6820 · Severity: critical · CVSS 8.1 · Exploited in the wild · Published 2021-11-03

A race condition in Mozilla Firefox and Thunderbird when handling a ReadableStream can lead to a use-after-free vulnerability. This flaw allows for unspecified impacts and has been observed in targeted attacks in the wild.

Technologies: [Mozilla Firefox](https://junglewise.ai/threats/technologies/firefox), [Mozilla Thunderbird](https://junglewise.ai/threats/technologies/thunderbird). Vendors: [Mozilla](https://junglewise.ai/threats/vendors/mozilla).

## Quick answers

**Is CVE-2020-6820 exploited in the wild?** Yes. CVE-2020-6820 is known to be exploited in the wild, first recorded on 3 November 2021. Treat it as urgent.

**How severe is CVE-2020-6820?** Critical severity, with a CVSS score of 8.1, published 3 November 2021.

**Which products and versions are affected?** Mozilla Firefox < 74.0.1; Mozilla Firefox ESR < 68.6.1; Mozilla Thunderbird < 68.7.0.

**Is there a fix for CVE-2020-6820?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2020-6820 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2020-6820?** Check whether you run Mozilla Firefox or Mozilla Thunderbird. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Affected products

- Mozilla Firefox < 74.0.1
- Mozilla Firefox ESR < 68.6.1
- Mozilla Thunderbird < 68.7.0

## Timeline

- 2020-04-24: disclosed: NVD Published Date
- 2021-11-03: kev added: Date added to CISA Known Exploited Vulnerabilities Catalog
- 2021-11-03: exploited: Reported as exploited in the wild in CISA KEV catalog and vendor advisory.

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