CVE-2025-6426 is a vulnerability identified in Mozilla Firefox and Thunderbird specifically on macOS platforms, affecting Firefox versions prior to 140 and Thunderbird versions prior to 140 (including ESR versions below 128.12). The core issue is that the browser does not display a warning prompt when opening files with the .terminal extension, which are executable terminal script files on macOS. This lack of warning can lead users to inadvertently execute malicious scripts, resulting in arbitrary code execution. The vulnerability is classified under CWE-345 (Insufficient Verification of Data Authenticity), indicating that the application fails to properly verify or warn about potentially dangerous file types before execution. The CVSS v3.1 base score is 8.8, indicating a high-severity vulnerability with network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:N), user interaction required (UI:R), unchanged scope (S:U), and high impact on confidentiality, integrity, and availability (C:H/I:H/A:H). Although no public exploits have been reported yet, the vulnerability poses a significant risk due to the potential for remote attackers to trick users into opening malicious .terminal files, leading to full system compromise. This vulnerability affects only macOS versions of Firefox and Thunderbird, with other platforms unaffected. The absence of patch links suggests that fixes may be forthcoming or pending release. Organizations relying on Firefox and Thunderbird on macOS should monitor for updates and prepare to deploy patches promptly.

For European organizations, this vulnerability presents a critical risk primarily to users running macOS versions of Firefox or Thunderbird. Successful exploitation can lead to arbitrary code execution, allowing attackers to compromise user systems, steal sensitive information, manipulate data, or disrupt operations. Given the high confidentiality, integrity, and availability impacts, this could result in data breaches, loss of trust, operational downtime, and potential regulatory penalties under GDPR if personal data is exposed. The requirement for user interaction (opening a .terminal file) means phishing or social engineering campaigns could be leveraged to exploit this vulnerability. Organizations with remote or hybrid workforces using macOS devices are particularly vulnerable. The lack of current known exploits provides a window for proactive mitigation, but the high CVSS score underscores the urgency. Additionally, the vulnerability affects both Firefox and Thunderbird, which are widely used for web browsing and email communication, increasing the attack surface. The impact is amplified in sectors with high security requirements such as finance, government, healthcare, and critical infrastructure.

1. Immediately inventory and identify all macOS systems running affected versions of Firefox and Thunderbird. 2. Monitor Mozilla security advisories closely and apply official patches or updates as soon as they are released to address CVE-2025-6426. 3. In the interim, configure endpoint protection solutions to block or quarantine files with the .terminal extension received via email or downloaded from the internet. 4. Implement strict email filtering and attachment scanning to detect and block suspicious .terminal files or phishing attempts. 5. Educate users about the risks of opening unknown or unexpected .terminal files, emphasizing caution with files received from untrusted sources. 6. Consider deploying application control or whitelisting solutions to prevent execution of unauthorized terminal scripts. 7. Review and tighten macOS security settings to restrict execution of scripts and binaries from untrusted locations. 8. Encourage use of alternative browsers or email clients on macOS until patches are applied if feasible. 9. Conduct phishing simulation exercises to raise awareness about social engineering tactics that could exploit this vulnerability. 10. Maintain robust backup and incident response plans to quickly recover from potential exploitation.