CVE-2026-16399 is a site isolation issue within the DOM: Navigation component of Mozilla Firefox. Site isolation is a security feature designed to separate web content from different origins into distinct processes to prevent cross-origin data leaks. This vulnerability could allow bypassing these isolation boundaries, potentially exposing sensitive information. The issue was fixed in Firefox 153 as part of a broader set of security fixes addressing multiple high-impact vulnerabilities. The CVSS 3.1 vector indicates the vulnerability can be exploited remotely without privileges or user interaction, impacting confidentiality but not integrity or availability.

Successful exploitation of this vulnerability could allow an attacker to bypass site isolation protections, potentially leading to unauthorized access to sensitive information from other origins within the browser context. The CVSS score of 7.5 reflects a high impact on confidentiality. There is no indication of impact on integrity or availability. No known exploits in the wild have been reported at this time.

Mozilla has released an official fix for this vulnerability in Firefox version 153. Users and administrators should update affected Firefox installations to version 153 or later to remediate this issue. Since this is not a cloud service, manual updating is required. No additional mitigation steps are indicated by the vendor advisory.