Mozilla Firefox And Thunderbird Use-After-Free 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.
- Vendor
- Mozilla
- Product
- Firefox and Thunderbird
- CVSS
- 8.1
- EPSS (exploit probability)
- 7.1%
- Status
- kev
- CISA patch-by (BOD 22-01)
- Added to KEV
Is CVE-2020-6820 being exploited?
Yes. CISA added CVE-2020-6820 to its Known Exploited Vulnerabilities catalog on . CISA only lists a CVE when it has reliable evidence of exploitation in the wild.
FIRST's EPSS model puts the probability of exploitation activity in the next 30 days at 7.1% (scored October 14, 2026).
- CISA KEV[ LISTED ]
- ENISA EUVD (exploited)[ — ]
- CIRCL KEV[ — ]
Catalog snapshot 2026-10-19. Full cross-catalog view: /kev-comparison
What to do
CISA required actionApply updates per vendor instructions.
US federal civilian agencies must remediate by (181 days after listing) under BOD 22-01. Everyone else: that deadline is a sensible upper bound, not a target. Vendor guidance and the NVD record are linked under Source above.
Other Mozilla Firefox and Thunderbird CVEs we track
| CVE / Vulnerability | Severity | CVSS | EPSS | Added to KEV |
|---|---|---|---|---|
| CVE-2016-9079 Mozilla Firefox, Firefox ESR, and Thunderbird Use-After-Free Vulnerability | [ HIGH ] | 7.5 | 87.4% | |
| CVE-2019-11708 Mozilla Firefox and Thunderbird Sandbox Escape Vulnerability | [ CRITICAL ] | 10.0 | 55.9% | |
| CVE-2019-11707 Mozilla Firefox and Thunderbird Type Confusion Vulnerability | [ HIGH ] | 8.8 | 37.7% | |
| CVE-2013-1690 Mozilla Firefox and Thunderbird Denial-of-Service Vulnerability | [ HIGH ] | 8.8 | 69.0% | |
| CVE-2020-6819 Mozilla Firefox And Thunderbird Use-After-Free Vulnerability | [ HIGH ] | 8.1 | 3.0% | |
| CVE-2019-17026 Mozilla Firefox And Thunderbird Type Confusion Vulnerability | [ HIGH ] | 8.8 | 46.3% |