CVE-2026-16381 is a critical security vulnerability in Mozilla Firefox's Networking: DNS component that enables bypass of the same-origin policy, a fundamental browser security mechanism. This flaw was resolved in Firefox 153 and Firefox ESR 140.13. The vulnerability has a CVSS 3.1 base score of 9.1 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N), indicating it can be exploited remotely without privileges or user interaction, leading to high confidentiality and integrity impacts. Mozilla's official security advisories (MFSA 2026-68 and MFSA 2026-70) confirm the fix and provide detailed references. Multiple other high-impact vulnerabilities were also addressed in these releases. There are no confirmed active exploits in the wild at this time.

Successful exploitation of this vulnerability could allow an attacker to bypass the same-origin policy in Firefox's DNS networking component, potentially enabling unauthorized access to sensitive information or manipulation of web content across origins. The CVSS score of 9.1 reflects critical impact on confidentiality and integrity without requiring user interaction or privileges. However, no active exploitation in the wild has been reported.

Mozilla has released official fixes for this vulnerability in Firefox version 153 and Firefox ESR version 140.13. Users and administrators should update affected Firefox installations to these versions or later to remediate the issue. Since this is not a cloud service, patching the client software is required. There are no vendor advisories indicating that no action is required or that the issue is already mitigated without patching.