CVE-2020-6820: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability
Vulnerability summary
CVE-2020-6820 is a high (CVSS 8.1) Race Condition vulnerability in Mozilla Firefox and Thunderbird. It is known to be exploited: CISA lists it, first on 03-11-2021. EPSS estimates a 7.06% chance of exploitation in the next 30 days, higher than 94.1% of scored vulnerabilities.
Advisory lineage
Upstream 0 · Downstream 27 · Related 13Timeline
- 24-04-2020PublishedVulnerability first disclosed.
- 03-11-2021Added to CISA KEVMozilla Firefox And Thunderbird Use-After-Free Vulnerability
- 03-05-2022CISA remediation dueApply updates per vendor instructions.
- 21-10-2025Last modifiedVulnerability information updated.
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.
CVSS metrics
5 scores · 4 sources · 2 versions- OSV · UbuntuEcosystem advisory · calculated8.1HighCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
- OSV · DebianEcosystem advisory · calculated8.1HighCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
- NVDNIST8.1HighCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS trend
Probability
7.06%
Chance of exploitation in the next 30 days
Percentile
94%
Versus all other CVEs
30-day change
0.00 pp
From 7.06% on 19-09-2026
Known exploited
Compare KEV catalogs
CISA KEVListed- Added
- 03-11-2021
- Federal due date
- 03-05-2022
CISA required action
Apply updates per vendor instructions. nvd.nist.gov/vuln/detail/CVE-2020-6820
Public exploits
0 foundNo public exploits indexed.
Weaknesses
CWE · root causeAffected systems
Mozilla3 products
- firefox≥ unspecified, < 74.0.1 | < 68.6.1 | < 74.0.1
- firefox_esr≥ unspecified, < 68.6.1
- thunderbird≥ unspecified, < 68.7.0 | < 68.7.0
Debian2 products
- firefox-esr< 68.6.1esr-1
- thunderbird< 1:68.7.0-1
Ubuntu2 products
- firefox< 74.0.1+build1-0ubuntu0.16.04.1 | < 74.0.1+build1-0ubuntu0.18.04.1 | < 75.0+build3-0ubuntu1
- thunderbird< 1:68.7.0+build1-0ubuntu0.16.04.2 | < 1:68.7.0+build1-0ubuntu0.18.04.1 | < 1:68.7.0+build1-0ubuntu1
3 vendors · 7 products
References
13 links- www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2020-6820Government ResourceUS Government Resource
