CVE-2026-74960: Site isolation issue in the WebExtensions component

Severity: high (CVSS 8.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-08-18 · updated 2026-08-24
Weakness: CWE-284

Description

Site isolation issue in the WebExtensions component. This vulnerability was fixed in Firefox 154, Firefox ESR 140.14, Firefox ESR 153.1, Thunderbird 154, Thunderbird 140.14, and Thunderbird 153.1.

Affected products

Mozilla Firefox: before 140.14.0; from 141.0 before 153.1.0 — fixed in 140.14.0, 153.1.0
Mozilla Thunderbird: before 140.14.0; from 141.0 before 153.1.0 — fixed in 140.14.0, 153.1.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 140.14.0, 153.1.0; Mozilla Thunderbird 140.14.0, 153.1.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/show_bug.cgi?id=2049148
https://www.mozilla.org/security/advisories/mfsa2026-74/
https://www.mozilla.org/security/advisories/mfsa2026-76/
https://www.mozilla.org/security/advisories/mfsa2026-77/
https://www.mozilla.org/security/advisories/mfsa2026-78/
https://www.mozilla.org/security/advisories/mfsa2026-79/
https://www.mozilla.org/security/advisories/mfsa2026-80/

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