CVE-2020-6820

Q: How do I fix CVE-2020-6820?

Published advisories record a fix for 6 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.

Q: Is CVE-2020-6820 being actively exploited?

Yes. CVE-2020-6820 is listed in the CISA Known Exploited Vulnerabilities catalog, which means exploitation has been observed in the wild. US federal agencies were required to remediate it by 2022-05-03. Treat it as a priority whatever its CVSS score says.

Q: How severe is CVE-2020-6820?

CVE-2020-6820 has a CVSS v3 base score of 8.1, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

Q: What does CVE-2020-6820 affect?

Published advisories record a fix for firefox (Debian), firefox (Ubuntu), thunderbird (Ubuntu), DeveloperEdition (Windows application), and 2 more. Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

HIGHCISA KEVEPSS 93th pctl

8.1

CVSS v3

NVD

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.

CWE-362Race Condition

How to fix

Remediation Available

firefoxDebian

Fixed in:74.0.1-1CVE-2020-6820

firefoxUbuntu

Fixed in:74.0.1+build1-0ubuntu0.16.04.1USN-4317-1

Fixed in:74.0.1+build1-0ubuntu0.18.04.1USN-4317-1

thunderbirdUbuntu

Fixed in:1:68.7.0+build1-0ubuntu0.16.04.2USN-4335-1

Fixed in:1:68.7.0+build1-0ubuntu0.18.04.1USN-4328-1

DeveloperEditionWindows application

Affected:74.0.1Fixed in:74.0.1Mozilla

Affected:68.6.1Fixed in:68.6.1Mozilla

Mozilla Firefox (en-US)Windows application

Affected:74.0.1Fixed in:74.0.1Mozilla

Affected:68.6.1Fixed in:68.6.1Mozilla

Mozilla Thunderbird (en-US)Windows application

Affected:68.7.0Fixed in:68.7.0Mozilla

TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityHigh

Privileges RequiredNone

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityHigh

IntegrityHigh

AvailabilityHigh

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

Exploit Intelligence

6.30%probability of exploitation in 30 days

93rdpercentile

High risk: more likely to be exploited than 93% of all known CVEs.

Known Exploited Vulnerability (CISA KEV)

Mozilla Firefox And Thunderbird Use-After-Free Vulnerability

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.

Apply updates per vendor instructions.

Remediation due: 2022-05-03

References

Vendor Advisory2

Third-Party Advisory1

usn.ubuntu.com

Government Resource1

cisa.gov

Issue Tracking1

bugzilla.mozilla.org

Related Vulnerabilities

Other CWE-362 (Race Condition) vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2024-6387	High	8.1	100%	-	Fix
CVE-2023-36884	High	7.5	99%	KEV + Ransom	Fix
CVE-2018-15473	Medium	5.3	99%	-	Fix
CVE-2024-27983	High	8.2	87%	-	Fix
CVE-2014-0226	Medium	6.8	86%	-	Fix
CVE-2016-5195	High	7.0	84%	KEV	Fix

Common questions

How do I fix CVE-2020-6820?

Published advisories record a fix for 6 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.

Is CVE-2020-6820 being actively exploited?

Yes. CVE-2020-6820 is listed in the CISA Known Exploited Vulnerabilities catalog, which means exploitation has been observed in the wild. US federal agencies were required to remediate it by 2022-05-03. Treat it as a priority whatever its CVSS score says.

How severe is CVE-2020-6820?

CVE-2020-6820 has a CVSS v3 base score of 8.1, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

What does CVE-2020-6820 affect?

Published advisories record a fix for firefox (Debian), firefox (Ubuntu), thunderbird (Ubuntu), DeveloperEdition (Windows application), and 2 more. Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2025-11-04.