What is CVE-2026-12309?

CVE-2026-12309 is a medium-severity vulnerability in Mozilla Firefox, classified under Improper Restriction of Operations within the Bounds of a Memory Buffer. CVSS score: 6.5/10. Published 2026-06-16.

How severe is CVE-2026-12309?

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

Buffer overflow in Mozilla Firefox

CVE-2026-12309

Memory safety bug fixed in Firefox 152. This vulnerability was fixed in Firefox 152, Firefox ESR 140.12, Thunderbird 152, and Thunderbird 140.12.

Vulnerability class: Buffer Overflow

Published 2026-06-16 · last modified 2026-06-18.

CVSS v3 metric

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

EPSS exploit prediction

EPSS: 0.002 (13.0th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 13th percentile — 13.0% 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

Mozilla Firefox — versions 140.12, 152
Mozilla Thunderbird — versions 140.12, 152

Weakness classification (CWE)

CWE-119 — Improper Restriction of Operations within the Bounds of a Memory Buffer

References

bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
www.mozilla.org/security/advisories/mfsa2026-57/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-58/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-60/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-61/ (Vendor Advisory)

Frequently asked questions

What is CVE-2026-12309?: CVE-2026-12309 is a medium-severity vulnerability in Mozilla Firefox, classified under Improper Restriction of Operations within the Bounds of a Memory Buffer. CVSS score: 6.5/10. Published 2026-06-16.
How severe is CVE-2026-12309?: Medium severity. CVSS v3 base score is 6.5 out of 10.