CVE-2020-6820

KEVMozilla Firefox And Thunderbird Use-After-Free Vulnerability

Description

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.

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 3.1 vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

PublishedApr 24, 2020

Last modifiedJun 17, 2026

Added to KEVNov 3, 2021

Remediation dueMay 3, 2022

References

bugzilla.mozilla.org/show_bug.cgi?id=1626728Issue TrackingPermissions Required
usn.ubuntu.com/4335-1/Third Party Advisory
mozilla.org/security/advisories/mfsa2020-11/Vendor Advisory
mozilla.org/security/advisories/mfsa2020-14/Vendor Advisory
cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2020-6820US Government Resource

0 recent articles mention this CVE

Risk assessment · High

Actively exploited. Patch immediately.

Exploitation: Confirmed in the wild (CISA KEV)
Federal remediation deadline: Overdue by 1637 days
CVSS: 8.1 HIGH
EPSS (30-day exploit probability): 7.1% · top 6% of all CVEs
Weakness: CWE-362
Attack vector: Network, remotely reachable
Preconditions: No privileges and no user interaction

Recommended actions

1.Apply updates per vendor instructions.
2.Treat as emergency change: patch or isolate internet-facing instances first.
3.Review exposure: confirm which instances are reachable from the internet and restrict access until patched.
4.Hunt for compromise: review logs for the disclosure window, not just from the patch date.

Assessment is derived automatically from the live signals above; verify against your own asset inventory.