What is CVE-2026-84136?

CVE-2026-84136 is a medium-severity vulnerability in Mozilla Firefox, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2026-09-01.

How severe is CVE-2026-84136?

Medium severity. CVSS v3 base score is 6.1 out of 10.

XSS in Mozilla Firefox

CVE-2026-84136

Other issue in the DOM: Navigation component. This vulnerability was fixed in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2.

Vulnerability class: XSS (Cross-Site Scripting)

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

CVSS v3 metric

CVSS v3 base score 6.1 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.002 (9.5th percentile), scored 2026-10-17.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 10th percentile — 9.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.002 · newest: 0.002 · 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-2026-84136?: CVE-2026-84136 is a medium-severity vulnerability in Mozilla Firefox, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2026-09-01.
How severe is CVE-2026-84136?: Medium severity. CVSS v3 base score is 6.1 out of 10.