CVE-2026-6774: 
NixOS vulnerability analysis and mitigation

CVE-2026-6774 is a mitigation bypass vulnerability in the DOM: Security component of Mozilla Firefox and Thunderbird. Discovered and reported by security researcher "lebr0nli," it was publicly disclosed on April 21, 2026, as part of Mozilla Foundation Security Advisory 2026-30. The vulnerability affects Firefox versions prior to 150.0 and Thunderbird versions prior to 150.0. It carries a CVSS v3.1 base score of 5.4 (Medium), as assessed by CISA-ADP (Mozilla Advisory, Github Advisory).

The vulnerability is classified under CWE-693 (Protection Mechanism Failure) and CWE-358 (Improperly Implemented Security Check for Standard), indicating that a browser security control within the DOM: Security component can be bypassed due to an incorrect or incomplete implementation (Github Advisory). Exploitation requires network access, low privileges, and user interaction, with a changed scope indicating that a successful bypass can affect resources outside the vulnerable component's security boundary. The Mozilla bug tracker entry (Bug 2016915) is marked as requiring permissions to view, so specific technical details of the bypass mechanism have not been publicly disclosed (Mozilla Advisory). Mozilla rated the impact of this specific CVE as "low" within the broader Firefox 150 advisory, distinguishing it from higher-severity issues in the same release.

Successful exploitation of this vulnerability could allow an attacker to bypass DOM security mitigations in Firefox or Thunderbird, resulting in limited confidentiality and integrity impacts (both rated "Low" in the CVSS assessment) with no availability impact. The changed scope indicates that the bypass could affect browser security boundaries, potentially enabling cross-origin data access or unauthorized content manipulation within the browser context. Exploitation requires a victim to interact with attacker-controlled content, limiting the attack surface to social engineering or malicious web pages (Github Advisory, Mozilla Advisory).

No public proof-of-concept exploit code or evidence of in-the-wild exploitation has been identified for CVE-2026-6774. The EPSS score is approximately 0.035–0.043%, placing it in the 14th percentile for exploitation likelihood within 30 days, indicating a low probability of active exploitation (Github Advisory). The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. No threat actor attribution has been reported.

Mozilla has addressed this vulnerability in Firefox 150 and Thunderbird 150, released on April 21, 2026. Users and administrators should update to Firefox 150 or later and Thunderbird 150 or later immediately. No configuration-based workarounds have been published; upgrading to the patched version is the only recommended remediation (Mozilla Advisory, Mozilla Advisory (Thunderbird)).

The CIS (Center for Internet Security) published an advisory noting that multiple vulnerabilities in Mozilla products fixed in Firefox 150 could allow for arbitrary code execution, grouping CVE-2026-6774 among the broader set of issues addressed in the release. Security news outlets including GBHackers, CyberPress, and CyberNoz covered the Firefox 150 release, highlighting the range of fixes. No specific researcher commentary or significant social media discussion focused exclusively on CVE-2026-6774 has been identified, consistent with its "low" impact rating within the release.