CVE-2026-74986: Site isolation issue in the CSS Parsing and Computation component

Severity: critical (CVSS 9.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-08-18 · updated 2026-08-25
Weakness: CWE-200

Description

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.

Affected products

Mozilla Firefox: before 153.1.0 — fixed in 153.1.0
Mozilla Thunderbird: before 153.1.0 — fixed in 153.1.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 153.1.0; Mozilla Thunderbird 153.1.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/show_bug.cgi?id=2060048
https://www.mozilla.org/security/advisories/mfsa2026-74/
https://www.mozilla.org/security/advisories/mfsa2026-77/
https://www.mozilla.org/security/advisories/mfsa2026-78/
https://www.mozilla.org/security/advisories/mfsa2026-80/

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