CVE-2020-6820

HIGH8.1

CISA KEV

Published 2020-04-24 · Updated 2026-06-17 · Source security@mozilla.org

Summary

CVE-2020-6820 is a high-severity (CVSS 8.1) vulnerability in Mozilla Firefox and Thunderbird. CISA added it to the Known Exploited Vulnerabilities catalog on November 3, 2021, which means exploitation in the wild has been confirmed; US federal agencies had to remediate it by May 3, 2022. CISA does not currently list it as used in ransomware campaigns. Mozilla has 13 vulnerabilities in the KEV catalog, 1 of them linked to ransomware.

All 13 known-exploited Mozilla vulnerabilities →

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.

CVSS vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

CWE-362CWE-362

CISA Known-Exploited Vulnerability

Product: Mozilla — Firefox and Thunderbird

Name: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability

Date added: 2021-11-03 · Due: 2022-05-03

Required action: Apply updates per vendor instructions.

References

View on NVD

More exploited Mozilla vulnerabilities

Most recently added to CISA KEV

CVE-2010-3765
Multiple Products — Mozilla Multiple Products Remote Code Execution Vulnerability
added 2025-10-06
CVE-2024-9680
RANSOMWARE
Firefox — Mozilla Firefox Use-After-Free Vulnerability
added 2024-10-15
CVE-2016-9079
Firefox, Firefox ESR, and Thunderbird — Mozilla Firefox, Firefox ESR, and Thunderbird Use-After-Free Vulnerability
added 2023-06-22
CVE-2015-4495
Firefox — Mozilla Firefox Security Feature Bypass Vulnerability
added 2022-05-25
CVE-2019-11707
Firefox and Thunderbird — Mozilla Firefox and Thunderbird Type Confusion Vulnerability
added 2022-05-23
CVE-2019-11708
Firefox and Thunderbird — Mozilla Firefox and Thunderbird Sandbox Escape Vulnerability
added 2022-05-23

View all →

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