High 8.1Known exploited (CISA KEV)Published Apr 24, 2020

CVE-2020-6820Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free.

Updated under an hour ago (Oct 26, 2026, 4:30 PM ET)

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It affects Mozilla Thunderbird and 2 other products. Add what you run to find out whether that includes you.

CISA lists it as known exploited since Nov 3, 2021; federal agencies had to remediate by May 3, 2022. Treat that date as your deadline.

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Exploited in the last 30 days

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: 8.1 HIGH (v3.1, CISA ADP)CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
Weakness (CWE): CWE-362
Known exploited: Added to CISA KEV Nov 3, 2021; federal remediation due May 3, 2022Apply updates per vendor instructions.
EPSS (exploitation in the next 30 days): 7.06% (percentile 94)
Assigned by: mozilla
NVD analysis: Analyzed
Record last updated: Oct 21, 2025
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

Mozilla Thunderbird mozilla:thunderbird (from the name)	from unspecified before 68.7.0	CNA record
Mozilla Firefox mozilla:firefox (from the name)	from unspecified before 74.0.1	CNA record
Mozilla Firefox ESR mozilla:firefox_esr (from the name)	from unspecified before 68.6.1	CNA record
mozilla firefox mozilla:firefox	before 68.6.1; before 74.0.1	NVD
mozilla thunderbird mozilla:thunderbird	before 68.7.0	NVD

References

References

https://www.mozilla.org/security/advisories/mfsa2020-14/	x-refsource-misc, vendor-advisory
https://www.mozilla.org/security/advisories/mfsa2020-11/	x-refsource-misc, vendor-advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1626728	x-refsource-misc, issue-tracking, permissions-required
https://usn.ubuntu.com/4335-1/	vendor-advisory, x-refsource-ubuntu, third-party-advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2020-6820	us-government-resource

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.