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

Published Apr 3, 2020
·
Updated

Last updated 25 August 2025

Other sources

Mozilla Firefox and Thunderbird contain a race condition vulnerability when handling a ReadableStream under certain conditions. The race condition creates a use-after-free vulnerability, causing unspecified impacts.

— CISA

Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free.

— Mozilla

Affected Software

12 affected componentsFixes available
Mozilla Firefox<74.0.1
74.0.1
Mozilla Firefox ESR<68.6.1
68.6.1
Mozilla Thunderbird<68.7
68.7
Mozilla Firefox<74.0.1
Mozilla Firefox ESR<68.6.1
Mozilla Thunderbird<68.7.0
Mozilla Firefox and Thunderbird
Mozilla Firefox<68.6.1
Mozilla Firefox<74.0.1
debian/firefox
152.0.6-1
debian/firefox-esr
115.14.0esr-1~deb11u1140.13.0esr-1~deb11u1140.12.0esr-1~deb12u1140.13.0esr-1~deb12u1140.12.0esr-1~deb13u1140.13.0esr-1~deb13u1140.12.0esr-1140.13.0esr-2
debian/thunderbird
1:115.12.0-1~deb11u11:140.12.0esr-1~deb11u11:140.12.0esr-1~deb12u11:140.12.0esr-1~deb13u11:140.12.0esr-1

Remediation

Recommended actions to resolve this vulnerability, in priority order.

  1. Upgrade

    Upgrade Firefox to a version that resolves this vulnerability.

    Fixed in 74.0.1
  2. Upgrade

    Upgrade Firefox ESR to a version that resolves this vulnerability.

    Fixed in 68.6.1
  3. Upgrade

    Upgrade Thunderbird to a version that resolves this vulnerability.

    Fixed in 68.7
  4. Upgrade

    Upgrade debian/firefox to a version that resolves this vulnerability.

    Fixed in 152.0.6-1
  5. Upgrade

    Upgrade debian/firefox-esr to a version that resolves this vulnerability.

    Fixed in 115.14.0esr-1~deb11u1Fixed in 140.13.0esr-1~deb11u1Fixed in 140.12.0esr-1~deb12u1Fixed in 140.13.0esr-1~deb12u1Fixed in 140.12.0esr-1~deb13u1Fixed in 140.13.0esr-1~deb13u1Fixed in 140.12.0esr-1Fixed in 140.13.0esr-2
  6. Upgrade

    Upgrade debian/thunderbird to a version that resolves this vulnerability.

    Fixed in 1:115.12.0-1~deb11u1Fixed in 1:140.12.0esr-1~deb11u1Fixed in 1:140.12.0esr-1~deb12u1Fixed in 1:140.12.0esr-1~deb13u1Fixed in 1:140.12.0esr-1
  7. Upgrade

    Upgrade Mozilla Firefox to a version that resolves this vulnerability.

    Fixed in 74.0.1
  8. Upgrade

    Upgrade Mozilla Firefox ESR to a version that resolves this vulnerability.

    Fixed in 68.6.1
  9. Upgrade

    Upgrade Mozilla Thunderbird to a version that resolves this vulnerability.

    Fixed in 68.7.0
  10. Compensating control

    Given targeted attacks in the wild, restrict exposure to untrusted content until all affected Firefox/Thunderbird versions are upgraded.

Event History

Apr 3, 2020
CVE Published
12:00 AM
Apr 24, 2020
CVE Published
via MITRE·03:56 PM
Data Sourced
via MITRE·03:56 PM
DescriptionWeakness
Data Sourced
via NVD·04:15 PM
DescriptionSeverityWeaknessAffected Software
Nov 3, 2021
Known Exploited
via CISA·12:00 AM
Jan 11, 2024
Data Sourced
via Launchpad·11:52 PM
Description
Feb 23, 2026
Data Sourced
via Ubuntu·03:19 PM
RemedyDescriptionSeverityAffected Software
Jul 24, 2026
Data Sourced
via Debian·04:21 AM
DescriptionAffected Software

Parent advisories

This vulnerability appears in the following advisories.

Peer vulnerabilities

Found alongside the following vulnerabilities.

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Frequently Asked Questions

1

What is CVE-2020-6820?

CVE-2020-6820 is a use-after-free vulnerability in Mozilla Firefox and Thunderbird.

2

Which software is affected by CVE-2020-6820?

Mozilla Firefox < 74.0.1, Firefox ESR < 68.6.1, and Thunderbird < 68.7.0 are affected.

3

How severe is CVE-2020-6820?

CVE-2020-6820 has a severity rating of 8.1 (critical).

4

How do I fix CVE-2020-6820 in Mozilla Firefox?

Update Mozilla Firefox to version 74.0.1 or higher to fix CVE-2020-6820.

5

How do I fix CVE-2020-6820 in Thunderbird?

Upgrade Thunderbird to version 68.7.0 or higher to fix CVE-2020-6820.

Company

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