Is CVE-2020-6820 in the CISA KEV catalog?

Yes. CVE-2020-6820 is on the CISA Known Exploited Vulnerabilities list, added 2021-11-03.

What is the CVSS severity of CVE-2020-6820?

CVE-2020-6820 has a CVSS v3 base score of 8.1 (HIGH).

What is the EPSS score for CVE-2020-6820?

CVE-2020-6820 has an EPSS score of 6.30% (93th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2020-6820?

Upgrade firefox on debian unstable to 74.0.1-1 or later. Fixes are available for 6 affected products.

CVE & CISA-KEV Catalog

CVE-2020-6820

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

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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.