CVE-2026-16387: Site isolation issue in the Networking component

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-07-21 · updated 2026-10-24
Weakness: CWE-200

Description

Site isolation issue in the Networking component. This vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.

Affected products

Mozilla Firefox: before 140.13.0; before 153.0.0 — fixed in 140.13.0, 153.0.0
Mozilla Thunderbird: before 140.13.0; from 141.0 before 153.0 — fixed in 140.13.0, 153.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 140.13.0, 153.0.0; Mozilla Thunderbird 140.13.0, 153.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/show_bug.cgi?id=2043200
https://www.mozilla.org/security/advisories/mfsa2026-68/
https://www.mozilla.org/security/advisories/mfsa2026-70/
https://www.mozilla.org/security/advisories/mfsa2026-71/
https://www.mozilla.org/security/advisories/mfsa2026-72/

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