CVE-2026-84121: Use-After-Free (CVSS 9.6)

Description

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.

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CVSS v3.1 Score

9.6

CRITICAL

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H

EPSS — Exploit Prediction

0.0032

Probability of exploitation

0.25%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-416 Use After Free

References

Other References

https://bugzilla.mozilla.org/show_bug.cgi?id=2059018 https://www.mozilla.org/security/advisories/mfsa2026-82/ https://www.mozilla.org/security/advisories/mfsa2026-83/ https://www.mozilla.org/security/advisories/mfsa2026-84/ https://www.mozilla.org/security/advisories/mfsa2026-85/ https://www.mozilla.org/security/advisories/mfsa2026-86/ https://www.mozilla.org/security/advisories/mfsa2026-87/ https://www.mozilla.org/security/advisories/mfsa2026-88/

Frequently Asked Questions

What is 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. It has a CVSS v3.1 base score of 9.6 (CRITICAL).

How severe is CVE-2026-84121? +

CVE-2026-84121 has a CVSS v3.1 score of 9.6 out of 10, rated CRITICAL. This is a critical vulnerability that should be patched immediately. The EPSS score is 0.0032, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-84121? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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