What is CVE-2020-6820?

CVE-2020-6820 is a high-severity vulnerability in Mozilla Firefox, classified under Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition). CVSS score: 8.1/10. Published 2020-04-24.

How severe is CVE-2020-6820?

High severity. CVSS v3 base score is 8.1 out of 10.

Is CVE-2020-6820 known to be exploited?

Yes. CVE-2020-6820 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), indicating it is being actively exploited. 2 public proof-of-concept repositories are indexed.

Vulnerability in Mozilla Firefox

CVE-2020-6820

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.

Vulnerability class: Race Condition

Published 2020-04-24 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 8.1 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.071 (94.1th percentile), scored 2026-10-19.

Low probability of exploitation in the next 30 days, but still tracked. 94th percentile — 94.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.071 · newest: 0.071 · change: 0.000

Affected products

Mozilla Firefox — versions unspecified
Mozilla Firefox Esr — versions unspecified
Mozilla Thunderbird — versions unspecified

Weakness classification (CWE)

CWE-362 — Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition)

CISA KEV (Known Exploited Vulnerabilities)

This CVE is on the CISA KEV catalog, added on 2021-11-03. CISA KEV inclusion means CISA has confirmed in-the-wild exploitation; US federal agencies are required to remediate within a published due date.

BOD 22-01 due date: 2022-05-03.

Required action: Apply updates per vendor instructions.

Public proof-of-concept exploits

References

www.mozilla.org/security/advisories/mfsa2020-14/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2020-11/ (Vendor Advisory)
bugzilla.mozilla.org/show_bug.cgi (Permissions Required, Issue Tracking)
usn.ubuntu.com/4335-1/ (vendor-advisory, Third Party Advisory)
134c704f-9b21-4f2e-91b3-4a467353bcc0 (US Government Resource)

Frequently asked questions

What is CVE-2020-6820?: CVE-2020-6820 is a high-severity vulnerability in Mozilla Firefox, classified under Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition). CVSS score: 8.1/10. Published 2020-04-24.
How severe is CVE-2020-6820?: High severity. CVSS v3 base score is 8.1 out of 10.
Is CVE-2020-6820 known to be exploited?: Yes. CVE-2020-6820 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), indicating it is being actively exploited. 2 public proof-of-concept repositories are indexed.