CVE-2026-16387 is a moderate-impact site isolation vulnerability affecting the Networking component of Mozilla Firefox. Site isolation issues can potentially allow processes to access data across site boundaries, undermining security boundaries within the browser. This vulnerability was fixed in Firefox 153 and Firefox ESR 140.13 as part of a comprehensive security update addressing multiple vulnerabilities across various components including DOM, JavaScript engine, graphics, and networking. The Mozilla advisories explicitly list this CVE with moderate impact and confirm remediation in the stated versions.

The vulnerability could allow bypass of site isolation protections in the Networking component, potentially leading to unauthorized access to data across site boundaries within the browser. This undermines the browser's security model but is rated moderate impact by the vendor. There are no reports of active exploitation in the wild for this specific issue.

Mozilla has released official fixes for this vulnerability in Firefox 153 and Firefox ESR 140.13. Users and administrators should update to these versions or later to remediate this issue. Since this is not a cloud service, remediation depends on applying these browser updates. Patch status is confirmed by the vendor advisory.