What is CVE-2026-74986?

CVE-2026-74986 is a critical-severity vulnerability in Mozilla Firefox, classified under Information Disclosure. CVSS score: 9.1/10. Published 2026-08-18.

How severe is CVE-2026-74986?

Critical severity. CVSS v3 base score is 9.1 out of 10.

Information disclosure in Mozilla Firefox

CVE-2026-74986

Site isolation issue in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox 154, Firefox ESR 153.1, Thunderbird 154, and Thunderbird 153.1.

Vulnerability class: Information Disclosure

Published 2026-08-18 · last modified 2026-08-25.

CVSS v3 metric

CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H.

EPSS exploit prediction

EPSS: 0.006 (44.2th percentile), scored 2026-10-02.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 44th percentile — 44.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.003 · newest: 0.006 · change: +0.002

Affected products

Weakness classification (CWE)

CWE-200 — Information Disclosure

References

bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
www.mozilla.org/security/advisories/mfsa2026-74/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-77/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-78/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-80/ (Vendor Advisory)

Frequently asked questions

What is CVE-2026-74986?: CVE-2026-74986 is a critical-severity vulnerability in Mozilla Firefox, classified under Information Disclosure. CVSS score: 9.1/10. Published 2026-08-18.
How severe is CVE-2026-74986?: Critical severity. CVSS v3 base score is 9.1 out of 10.