CVE-2026-84121

CVE-2026-84121

Sandbox escape due to use-after-free in the DOM: Security component. This vulnerability was fixed in Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, Firefox ESR 153.2, Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

Vulnerability class: Use-After-Free

CVSS v3 metric

CVSS v3 base score 9.6 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H.

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-84121?
CVE-2026-84121 is a critical-severity vulnerability, classified under Use After Free. CVSS score: 9.6/10. Published 2026-09-01.
How severe is CVE-2026-84121?
Critical severity. CVSS v3 base score is 9.6 out of 10.