CVE-1999-1076 is a vulnerability in the idle locking function of MacOS 9, an operating system released by Apple in the late 1990s. The flaw allows local users to bypass the password protection mechanism intended to secure idled sessions. Specifically, when a session is locked due to inactivity, a user attempting to log out can exploit this vulnerability by selecting the "Log Out" option and then choosing "Cancel" in the confirmation dialog box that appears when an application tries to verify the logout intent. Instead of logging out or remaining locked, the system erroneously returns the user to the previously locked session without requiring password authentication. This behavior effectively negates the security benefit of session locking, allowing unauthorized access to the session by anyone with local access to the machine. The vulnerability is classified with a CVSS score of 4.6 (medium severity), reflecting that it requires local access (AV:L), low attack complexity (AC:L), no authentication (Au:N), and impacts confidentiality, integrity, and availability to some extent (C:P/I:P/A:P). There is no patch available for this vulnerability, and no known exploits have been reported in the wild. Given the age of MacOS 9 and the nature of the vulnerability, it primarily affects legacy systems still running this outdated OS version.

For European organizations, the direct impact of CVE-1999-1076 is minimal in modern contexts because MacOS 9 is an obsolete operating system no longer in commercial or enterprise use. However, in niche environments where legacy MacOS 9 systems remain operational—such as in certain industrial, research, or archival settings—this vulnerability could allow unauthorized local users to bypass session locks and gain access to sensitive information or perform unauthorized actions. This could lead to confidentiality breaches, data integrity issues, and potential disruption of availability if malicious actions are taken within the unlocked session. The risk is primarily local, requiring physical or local access to the affected machine, which limits the scope of exploitation. Nonetheless, organizations with legacy Mac infrastructure should be aware of this risk, especially if these systems contain sensitive data or are connected to broader networks.

Given that no patch is available for this vulnerability and the affected system is MacOS 9, mitigation focuses on compensating controls. Organizations should: 1) Physically secure legacy MacOS 9 machines to prevent unauthorized local access, including locking rooms or cabinets where these systems reside. 2) Limit user access to these machines strictly to trusted personnel. 3) Consider isolating legacy systems from networks to reduce the risk of lateral movement if an attacker gains local access. 4) Use additional third-party security tools or screen locking utilities compatible with MacOS 9 that may provide more robust session locking mechanisms. 5) Plan and prioritize migration away from MacOS 9 to supported operating systems to eliminate exposure to this and other legacy vulnerabilities. 6) Implement strict monitoring and auditing of physical access to legacy systems to detect unauthorized attempts to access locked sessions.