What is CVE-2026-84138?

CVE-2026-84138 is a medium-severity vulnerability in Mozilla Firefox, classified under Uncontrolled Resource Consumption. CVSS score: 6.5/10. Published 2026-09-01.

How severe is CVE-2026-84138?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Resource exhaustion in Mozilla Firefox

CVE-2026-84138

Denial-of-service in the PDF Viewer component. This vulnerability was fixed in Firefox 155 and Thunderbird 155.

Vulnerability class: DoS (Denial of Service)

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

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H.

EPSS exploit prediction

EPSS: 0.004 (29.6th percentile), scored 2026-10-17.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 30th percentile — 29.6% 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-400 — Uncontrolled Resource Consumption

References

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

Frequently asked questions

What is CVE-2026-84138?: CVE-2026-84138 is a medium-severity vulnerability in Mozilla Firefox, classified under Uncontrolled Resource Consumption. CVSS score: 6.5/10. Published 2026-09-01.
How severe is CVE-2026-84138?: Medium severity. CVSS v3 base score is 6.5 out of 10.