CVE-2020-6820

Name: CVE-2020-6820
Creator: NVD / NIST
Published: 2020-04-24T16:15:13.463+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 8.1/10Actively ExploitedEPSS 6.30%

Last modified Jun 17, 2026

CVE-2020-6820 is a high-severity vulnerability rated 8.1/10 on the CVSS scale. 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. CISA has confirmed active exploitation in the wild. EPSS estimates a 6.30% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

8.1/10

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

EPSS Probability

6.30%

92.7th percentile

Probability of exploitation in the next 30 days. Learn more

Exploitation Status

This vulnerability is listed in CISA’s Known Exploited Vulnerabilities catalog, confirming active exploitation in the wild. Federal agencies must remediate by May 3, 2022.

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 68.6.1
Mozilla	Firefox	< 74.0.1
Mozilla	Thunderbird	< 68.7.0

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1626728Issue Tracking, Permissions Required
https://usn.ubuntu.com/4335-1/Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2020-11/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2020-14/Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1626728Issue Tracking, Permissions Required
https://usn.ubuntu.com/4335-1/Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2020-11/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2020-14/Vendor Advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2020-6820US Government Resource

Timeline

Published: Apr 24, 2020
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2020-6820?

How severe is CVE-2020-6820?

CVE-2020-6820 has a CVSS score of 8.1/10 (HIGH severity). The EPSS model estimates a 6.30% probability of exploitation in the next 30 days. This vulnerability is listed in CISA's Known Exploited Vulnerabilities catalog.

How do I fix CVE-2020-6820?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2020-6820?

Run a free Strix scan to check your systems for this vulnerability.

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Source: NVD / NIST