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

> CVE-2020-6820: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability. CVSS 8.1 (CVSS 3.1), in CISA KEV.

Canonical: https://rewindex.com/cve/CVE-2020-6820
Last updated: 2025-10-21T23:35:45Z

- Published: 2020-04-24 15:56 UTC · Updated: 2025-10-21 23:35 UTC · Assigner: mozilla
- CVSS: 8.1 (CVSS 3.1) · Severity: High · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
- EPSS: 7.1%, 94th percentile (scored 2026-10-25)
- CISA KEV: yes, added 2021-11-03, federal due date 2022-05-03, ransomware use: Unknown

## 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.

## CISA required action

Apply updates per vendor instructions.

## Affected vendors and products

- Mozilla: Firefox
- Mozilla: Firefox ESR
- Mozilla: Thunderbird

## References

- [CVE record on cve.org](https://www.cve.org/CVERecord?id=CVE-2020-6820)
- [NVD entry](https://nvd.nist.gov/vuln/detail/CVE-2020-6820)
- [https://bugzilla.mozilla.org/show\_bug.cgi?id=1626728](https://bugzilla.mozilla.org/show_bug.cgi?id=1626728)
- [https://usn.ubuntu.com/4335-1/](https://usn.ubuntu.com/4335-1/)
- [https://www.mozilla.org/security/advisories/mfsa2020-11/](https://www.mozilla.org/security/advisories/mfsa2020-11/)
- [https://www.mozilla.org/security/advisories/mfsa2020-14/](https://www.mozilla.org/security/advisories/mfsa2020-14/)

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_Sources: CVE records from the CVE Program (cve.org). CVE® is a registered trademark of The MITRE Corporation. (https://www.cve.org/) This product uses data from the NVD API but is not endorsed or certified by the NVD. (https://nvd.nist.gov/) Known Exploited Vulnerabilities catalog from CISA. (https://www.cisa.gov/known-exploited-vulnerabilities-catalog) EPSS scores from FIRST (Exploit Prediction Scoring System). (https://www.first.org/epss/)_


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Canonical: https://rewindex.com/cve/CVE-2020-6820 · Markdown: https://rewindex.com/cve/CVE-2020-6820.md · JSON: https://rewindex.com/api/v1/cves/CVE-2020-6820 · Data licensed CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) · Generated 2026-10-26T03:09:06.409Z