CVE-2026-6771 -- CVSS 9.8 Vulnerability Briefing

CVE-2026-6771 | CVSS 9.8 (Critical) | Exploit: No known exploit

What Is It

CVE-2026-6771 is a mitigation bypass vulnerability in the DOM Security component of Mozilla Firefox and Mozilla Thunderbird, allowing attackers to circumvent security controls enforced within the browser's document object model layer.

Technical Detail

The flaw resides in the DOM Security subsystem, where an attacker can craft malicious content that bypasses existing security mitigations intended to restrict or sandbox DOM-level operations. The precise bypass mechanism has not been fully disclosed, but vulnerabilities of this class typically allow an attacker to escape security boundaries enforced by the browser engine, potentially enabling cross-origin data access, content injection, or serving as a stepping stone toward code execution when chained with additional vulnerabilities. The CVSS score of 9.8 indicates the vulnerability is likely remotely exploitable without authentication and requires minimal or no user interaction beyond loading attacker-controlled content.

Exploitation Status

No known exploit has been publicly documented or confirmed as of April 28, 2026. This vulnerability is not currently listed in the CISA Known Exploited Vulnerabilities catalog. There is no evidence of active exploitation in the wild at this time, though the critical severity rating warrants prompt remediation regardless of current exploitation status.

Who Is Targeting This

No specific threat actor attribution at this time. No campaigns or targeted sectors have been associated with this vulnerability in available intelligence sources.

What To Do

Organizations should prioritize patching to Firefox 150 or Firefox ESR 140.10 and Thunderbird 150 or Thunderbird 140.10, as these releases contain the confirmed fix. Enterprise environments running managed Firefox or Thunderbird deployments should push updates immediately given the critical CVSS score. Where immediate patching is not feasible, consider restricting access to untrusted web content through network controls or browser policy configurations. Monitor endpoint management systems to confirm patch deployment coverage across all affected installations. No vendor-documented workaround is available as a substitute for patching.