CVE-2026-6771: 
NixOS vulnerability analysis and mitigation

CVE-2026-6771 is a mitigation bypass vulnerability in the DOM: Security component of Mozilla Firefox, Firefox ESR, Thunderbird, and Thunderbird ESR. Discovered and reported by Rayhan Hanaputra, it was publicly disclosed on April 21, 2026. Affected versions include Firefox before 150, Firefox ESR 140.x before 140.10, Thunderbird before 150, and Thunderbird ESR 140.x before 140.10. The vulnerability carries a CVSS v3.1 base score of 9.8 (Critical) as assessed by CISA-ADP, though Mozilla's own advisory rates its impact as moderate (Mozilla Advisory mfsa2026-30, Mozilla Advisory mfsa2026-32, Feedly).

The vulnerability is classified under CWE-288 (Authentication Bypass Using an Alternate Path or Channel) and CWE-358 (Improperly Implemented Security Check for Standard), indicating that an existing security control within the browser's DOM Security component can be circumvented via an alternate path or improperly validated check (Feedly). The flaw resides in the DOM: Security subsystem and allows an attacker to bypass browser-enforced security mitigations — such as same-origin policy enforcement or content security controls — without requiring user interaction or elevated privileges. The underlying bug is tracked at Mozilla Bugzilla ID 2025067, though the bug report is access-restricted (Mozilla Bugzilla). The attack vector is network-based with low complexity, meaning a remote attacker can exploit this through a crafted web page or HTML email.

Successful exploitation could allow a remote, unauthenticated attacker to bypass DOM security mitigations, potentially enabling unauthorized access to sensitive browser data, modification of page content or application state, and denial of service. The CISA-ADP assessment assigns high confidentiality, integrity, and availability impact, suggesting that in the worst case, exploitation could lead to significant data exposure or system disruption for affected users (Feedly). Because the vulnerability affects both Firefox and Thunderbird (including their ESR variants), the attack surface spans both web browsing and email client contexts, broadening the potential victim population.

As of the time of disclosure, there is no public proof-of-concept exploit and no evidence of in-the-wild exploitation (Feedly). The EPSS score is approximately 0.045%, reflecting a low near-term exploitation probability. The vulnerability is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. No threat actor attribution has been reported. The vulnerability requires no authentication and no user interaction, which lowers the bar for exploitation if a PoC were to become available.

Mozilla has released patches addressing CVE-2026-6771 in the following versions: Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird ESR 140.10 (Mozilla Advisory mfsa2026-30, Mozilla Advisory mfsa2026-32). Organizations should prioritize upgrading all Firefox and Thunderbird installations to the patched versions immediately. Downstream Linux distributions including Red Hat, Debian, SUSE, Oracle Linux, Rocky Linux, AlmaLinux, and Amazon Linux have also released updated packages. No configuration-based workaround is known; upgrading is the only effective remediation.

The CIS (Center for Internet Security) issued an advisory noting that multiple vulnerabilities in Mozilla products, including CVE-2026-6771, could allow for arbitrary code execution, recommending prompt patching (Feedly). Security news outlets including GBHackers, CyberPress, and Cybernoz covered the Firefox 150 release and its security fixes. The vulnerability received standard community attention on social platforms such as Bluesky and Mastodon, with no extraordinary controversy or debate noted. Kaspersky's threat intelligence portal also catalogued the vulnerability shortly after disclosure.