What is CVE-2026-84128?

CVE-2026-84128 is a high-severity vulnerability in Mozilla Firefox, classified under Improper Access Control. CVSS score: 8.8/10. Published 2026-09-01.

How severe is CVE-2026-84128?

High severity. CVSS v3 base score is 8.8 out of 10.

Vulnerability in Mozilla Firefox

CVE-2026-84128

Privilege escalation in the WebDriver BiDi component. This vulnerability was fixed in Firefox 155 and Thunderbird 155.

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

CVSS v3 metric

CVSS v3 base score 8.8 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.004 (36.5th percentile), scored 2026-10-17.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 37th percentile — 36.5% 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: 0.004 · change: +0.002

Affected products

Weakness classification (CWE)

CWE-284 — Improper Access Control

References

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

Frequently asked questions

What is CVE-2026-84128?: CVE-2026-84128 is a high-severity vulnerability in Mozilla Firefox, classified under Improper Access Control. CVSS score: 8.8/10. Published 2026-09-01.
How severe is CVE-2026-84128?: High severity. CVSS v3 base score is 8.8 out of 10.