CVE-1999-1076 in Mac OS
Summary
by MITRE
Idle locking function in MacOS 9 allows local users to bypass the password protection of idled sessions by selecting the "Log Out" option and selecting a "Cancel" option in the dialog box for an application that attempts to verify that the user wants to log out, which returns the attacker into the locked session.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability described in CVE-1999-1076 represents a critical security flaw in the macOS 9 operating system's session management mechanism. This issue specifically targets the idle locking functionality that was designed to protect user sessions when computers remained inactive for extended periods. The vulnerability exploits a fundamental weakness in the interaction between the system's lock screen and user logout dialog boxes, creating a bypass mechanism that allows unauthorized access to locked sessions.
The technical flaw manifests through a specific race condition and dialog handling issue within the macOS 9 environment. When a system enters an idle state and locks the session, the operating system presents a logout dialog box to applications that attempt to verify user intent before proceeding with logout operations. The vulnerability occurs when a local attacker selects the "Log Out" option from the locked session, but then immediately cancels the logout process through the dialog box. This sequence of actions, combined with the system's handling of the cancellation event, results in the session returning the user to the previously locked state without proper authentication verification.
This vulnerability directly relates to CWE-284 Access Control Bypass, which describes situations where a system fails to properly enforce access controls, allowing unauthorized users to gain access to protected resources. The flaw demonstrates how insufficient validation of user intent during session management operations can create persistent security gaps. The operational impact of this vulnerability extends beyond simple unauthorized access, as it represents a complete breakdown in the authentication mechanism that was specifically designed to protect user sessions from unauthorized access during periods of inactivity.
The attack vector for this vulnerability is straightforward and requires only local access to the affected system. An attacker with physical access or local user privileges can exploit this weakness to bypass password protection mechanisms that are supposed to safeguard user sessions. The vulnerability is particularly concerning because it operates at the system level, affecting core security functions rather than individual applications. This type of attack is classified under the ATT&CK technique T1547.001 Account Manipulation, which involves modifying or creating user accounts and privileges to maintain access to systems.
The mitigation strategies for this vulnerability require system administrators to implement comprehensive session management policies and ensure that all affected systems are properly updated. Organizations should consider implementing additional security measures such as automatic screen locking, enforced password policies, and monitoring for suspicious session activity. The vulnerability highlights the importance of proper input validation and dialog handling in security-critical systems. System updates and patches would need to address the specific race condition in the idle locking mechanism, ensuring that cancellation events properly validate user authentication status before returning users to locked sessions. This vulnerability serves as a reminder of the critical importance of thorough testing of security mechanisms, particularly those related to session management and user authentication flows.