CVE-2026-2775

CVE-2026-2775 is a mitigation bypass vulnerability in the DOM HTML Parser component of Mozilla Firefox and Thunderbird. It affects versions of Firefox prior to 148, Firefox ESR prior to 115.33 and 140.8, and Thunderbird prior to 148 and 140.8. The issue is classified under CWE-288 with a CVSS v3.1 base score of 9.8 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H), indicating critical severity due to its potential for high impact across confidentiality, integrity, and availability.

The vulnerability enables exploitation by any remote, unauthenticated attacker with network access and low attack complexity, requiring no user interaction or privileges. Successful exploitation allows the attacker to bypass security mitigations in the HTML parser, potentially leading to arbitrary code execution, data theft, or system compromise within the affected browser or email client context.

Mozilla's security advisories (MFSA 2026-13 through 2026-16) and the associated Bugzilla entry (bug 2015199) confirm the vulnerability was addressed in the listed fixed releases. Security practitioners should prioritize updating to Firefox 148 or later, Firefox ESR 115.33/140.8 or later, and Thunderbird 148/140.8 or later to mitigate the issue.