What is CVE-2026-84126?

CVE-2026-84126 is a medium-severity vulnerability in Mozilla Firefox, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 4.3/10. Published 2026-09-01.

How severe is CVE-2026-84126?

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

Buffer overflow in Mozilla Firefox

CVE-2026-84126

Incorrect boundary conditions in the Layout: Grid component. This vulnerability was fixed in Firefox 155 and Thunderbird 155.

Vulnerability class: Buffer Overflow

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:L/I:N/A:N.

EPSS exploit prediction

EPSS: 0.003 (22.5th percentile), scored 2026-10-17.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 23rd percentile — 22.5% 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: 0.003 · change: +0.001

Affected products

Weakness classification (CWE)

CWE-120 — Buffer Copy without Checking Size of Input (Classic Buffer Overflow)

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-2026-84126?: CVE-2026-84126 is a medium-severity vulnerability in Mozilla Firefox, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 4.3/10. Published 2026-09-01.
How severe is CVE-2026-84126?: Medium severity. CVSS v3 base score is 4.3 out of 10.