What is CVE-2026-8948?

CVE-2026-8948 is a critical-severity vulnerability in Mozilla Firefox, classified under Permissive Cross-domain Policy with Untrusted Domains. CVSS score: 9.1/10. Published 2026-05-19.

How severe is CVE-2026-8948?

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

XSS in Mozilla Firefox

CVE-2026-8948

Same-origin policy bypass in the DOM: Networking component. This vulnerability was fixed in Firefox 151 and Thunderbird 151.

Published 2026-05-19 · last modified 2026-07-15.

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:H/A:N.

EPSS exploit prediction

EPSS: 0.005 (36.8th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 37th percentile — 36.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.004 · newest: 0.005 · change: +0.000

Affected products

Mozilla Firefox — versions 151
Mozilla Thunderbird — versions 151

Weakness classification (CWE)

References

bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
www.mozilla.org/security/advisories/mfsa2026-46/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-50/ (Vendor Advisory)
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Frequently asked questions

What is CVE-2026-8948?: CVE-2026-8948 is a critical-severity vulnerability in Mozilla Firefox, classified under Permissive Cross-domain Policy with Untrusted Domains. CVSS score: 9.1/10. Published 2026-05-19.
How severe is CVE-2026-8948?: Critical severity. CVSS v3 base score is 9.1 out of 10.