CVE-2020-6820: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability
- Is it exploited?
Yes. CISA added it to the Known Exploited Vulnerabilities catalog on , 558 days after the CVE record was published.
- How likely is exploitation in the next 30 days?
- 7.1%
EPSS score, percentile 94.2 of all scored records
- How severe?
- 8.1 High
CVSS v3.1 base score, from CISA's enrichment of the record
- What should I do?
CISA deadline . Apply updates per vendor instructions.
- Which versions are affected?
Affected
- before 68.7.0
- before 74.0.1
- before 68.6.1
Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of ; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of ; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of . About the data.
What to do
Required action, set by CISA
Apply updates per vendor instructions.
CISA due date for this action: .
Advisories and patches
Linked from CISA's catalog entry
Affected products and versions
As listed in the CVE record.
| Status | Versions |
|---|---|
| Affected | before 68.7.0 |
| Status | Versions |
|---|---|
| Affected | before 74.0.1 |
| Status | Versions |
|---|---|
| Affected | before 68.6.1 |
Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of . About the data.
About this vulnerability
Mozilla Firefox and Thunderbird contain a race condition vulnerability when handling a ReadableStream under certain conditions. The race condition creates a use-after-free vulnerability, causing unspecified impacts.
Summary: CISA's KEV catalog, as of .
- CISA added CVE-2020-6820 to its Known Exploited Vulnerabilities (KEV) catalog on 3 Nov 2021.
- CISA's ransomware flag for this entry is "Unknown": no evidence of ransomware use in the catalog, which is not proof of none.
- The due date CISA sets for the required action is 3 May 2022.
- The CVE record was published on 24 Apr 2020, 558 days before it was added to KEV.
- The CVE record carries a CVSS v3.1 base score of 8.1 (High), taken from CISA's enrichment of the record.
- FIRST's EPSS model puts the probability of exploitation in the next 30 days at 7.1% (percentile 94.2), as of 28 Oct 2026.
- CISA's SSVC assessment in the CVE record: exploitation active, automatable no, technical impact total.
Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of ; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of ; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of . About the data.
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.
Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of . About the data.
Weakness
- CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.
Timeline
| Date | Event |
|---|---|
| CVE record published | |
| Added to CISA KEV | |
| CISA due date for the required action | |
| CVE record last updated |
References
More exploited Mozilla vulnerabilities
- CVE-2010-3765: Mozilla Multiple Products Remote Code Execution Vulnerability
- CVE-2024-9680: Mozilla Firefox Use-After-Free Vulnerability
- CVE-2016-9079: Mozilla Firefox, Firefox ESR, and Thunderbird Use-After-Free Vulnerability
- CVE-2015-4495: Mozilla Firefox Security Feature Bypass Vulnerability
- CVE-2019-11707: Mozilla Firefox and Thunderbird Type Confusion Vulnerability
- CVE-2019-11708: Mozilla Firefox and Thunderbird Sandbox Escape Vulnerability