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Known exploited · Mozilla

CVE-2020-6819: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability

What it is

Mozilla Firefox and Thunderbird contain a race condition vulnerability when running the nsDocShell destructor under certain conditions. The race condition creates a use-after-free vulnerability, causing unspecified impacts.

Added November 3, 2021Due May 3, 2022CISA KEV catalog 2026.10.01

Vendor
Mozilla
Product
Firefox and Thunderbird
Added to KEV
November 3, 2021
CISA due date
May 3, 2022
Ransomware use
Unknown

What CISA requires

Apply updates per vendor instructions.

Forensic triage

No

What to do now

  1. Find every instance of Firefox and Thunderbird you run, including the forgotten ones, and check its version against the vendor advisory.
  2. Apply the vendor's fix or mitigation, before May 3, 2022 if you are bound by CISA's deadline, sooner if it faces the internet.
  3. Assume it was tried against you already: look for signs of exploitation from the date it was added, not from the day you patched.

References

CVE-2020-6819 in the National Vulnerability Database

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Source: CISA Known Exploited Vulnerabilities Catalog (public domain), catalog version 2026.10.01.