How severe is CVE-2020-6820?

CVE-2020-6820 has a CVSS score of 8.1/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2020-6820?

Affected package information for CVE-2020-6820 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2020-6820?

No fixed version has been published for CVE-2020-6820 yet. Monitor the advisory for updates, apply mitigations recommended by the vendor, and use O3 Security to track when a fix becomes available in your stack.

Is CVE-2020-6820 actively exploited in the wild?

Yes. There are 3 known exploit references for CVE-2020-6820, including 3 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

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

CVE-2020-6820 has an EPSS (Exploit Prediction Scoring System) score of 3.1%, placing it in the 87th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

Is CVE-2020-6820 in the CISA Known Exploited Vulnerabilities catalog?

Yes. CVE-2020-6820 (Mozilla Firefox And Thunderbird Use-After-Free Vulnerability) was added to CISA's Known Exploited Vulnerabilities (KEV) catalog on 2021-11-03. CISA required all US federal civilian agencies to remediate this vulnerability by 2022-05-03. The required action was: Apply updates per vendor instructions. Being listed in the KEV catalog is the strongest possible confirmation of active exploitation — treat this as a critical priority.

When was CVE-2020-6820 published, and has it been updated?

CVE-2020-6820 was published on April 24, 2020 and was last updated on April 16, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CISA KEV·Added 2021-11-03 — agencies required to remediate by 2022-05-03

CVE-2020-6820

HIGH

Published

Apr 24, 2020

Updated

Apr 16, 2026

Affected

0 pkgs

Patched

None yet

Exploits

3 known

EPSS Exploitation Probability

via FIRST.org ↗

3.1%probability of exploitation in next 30 days

Lower Risk87th percentile0.00%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

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.

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

Under certain conditions, when handling a ReadableStream, a race conditi…

mozilla.orgGoogle-sheetApr 2020

Under certain conditions, when handling a ReadableStream, a race conditi…

nvd.nist.govNVDJul 2021

Under certain conditions, when handling a ReadableStream, a race conditi…

cisa.govCISAJul 2021

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2020-6820 in your stack?

O3 detects CVE-2020-6820 across dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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