What is CVE-2026-84137?

CVE-2026-84137 is a medium-severity vulnerability in Mozilla Firefox, classified under Origin Validation Error. CVSS score: 4.3/10. Published 2026-09-01.

How severe is CVE-2026-84137?

Medium severity. CVSS v3 base score is 4.3 out of 10.

Vulnerability in Mozilla Firefox

CVE-2026-84137

Spoofing issue in the DOM: Core & HTML component. This vulnerability was fixed in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2.

Published 2026-09-01 · last modified 2026-09-03.

CVSS v3 metric

CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N.

EPSS exploit prediction

EPSS: 0.001 (2.2th percentile), scored 2026-10-17.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 2nd percentile — 2.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.001 · newest: 0.001 · change: 0.000

Affected products

Weakness classification (CWE)

References

bugzilla.mozilla.org/show_bug.cgi (Permissions Required, Issue Tracking)
www.mozilla.org/security/advisories/mfsa2026-82/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-85/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-86/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-88/ (Vendor Advisory)

Frequently asked questions

What is CVE-2026-84137?: CVE-2026-84137 is a medium-severity vulnerability in Mozilla Firefox, classified under Origin Validation Error. CVSS score: 4.3/10. Published 2026-09-01.
How severe is CVE-2026-84137?: Medium severity. CVSS v3 base score is 4.3 out of 10.