CVE-2020-6819: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability
Vulnerability summary
CVE-2020-6819 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 3.04% chance of exploitation in the next 30 days, higher than 87.3% of scored vulnerabilities. 1 public exploit is recorded.
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 running the nsDocShell destructor, 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
3.04%
Chance of exploitation in the next 30 days
Percentile
87%
Versus all other CVEs
30-day change
0.00 pp
From 3.04% 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-2020-6819
Public exploits
1 foundExploit Reference (bugzilla.mozilla.org)
Maturity: unknown
Weaknesses
CWE · root cause- CWE-362Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.
- CWE-416Use After FreeThe product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
Affected 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-2020-6819Government ResourceUS Government Resource
