What is CVE-2026-16389?

CVE-2026-16389 is a critical-severity vulnerability in Mozilla Firefox, classified under Integer Overflow or Wraparound. CVSS score: 9.8/10. Published 2026-07-21.

How severe is CVE-2026-16389?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Integer overflow in Mozilla Firefox

CVE-2026-16389

Incorrect boundary conditions, integer overflow in the Libraries component in NSS. This vulnerability was fixed in Firefox 153 and Thunderbird 153.

Vulnerability class: Integer Overflow

Published 2026-07-21 · last modified 2026-08-10.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.006 (46.1th percentile), scored 2026-10-22.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 46th percentile — 46.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.006 · newest: 0.006 · change: 0.000

Affected products

Mozilla Firefox — versions 153
Mozilla Thunderbird — versions 153

Weakness classification (CWE)

CWE-190 — Integer Overflow or Wraparound

References

bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
www.mozilla.org/security/advisories/mfsa2026-68/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-71/ (Vendor Advisory)
af854a3a-2127-422b-91ae-364da2661108

Frequently asked questions

What is CVE-2026-16389?: CVE-2026-16389 is a critical-severity vulnerability in Mozilla Firefox, classified under Integer Overflow or Wraparound. CVSS score: 9.8/10. Published 2026-07-21.
How severe is CVE-2026-16389?: Critical severity. CVSS v3 base score is 9.8 out of 10.