CVE-2006-6127 in Mac OS X Server
Summary
by MITRE
Apple Mac OS X kernel allows local users to cause a denial of service via a process that uses kevent to register a queue and an event, then fork a child process that uses kevent to register an event for the same queue as the parent.
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Analysis
by VulDB Data Team • 04/29/2026
The vulnerability described in CVE-2006-6127 represents a critical flaw in the Apple Mac OS X kernel's handling of kqueue event notification mechanisms. This issue manifests when a parent process establishes a kqueue and registers events within it, subsequently forking a child process that attempts to register another event for the same kqueue. The kernel fails to properly manage this scenario, leading to system instability and potential denial of service conditions. The flaw exists at the kernel level where the event notification subsystem does not adequately validate or handle concurrent access patterns between parent and child processes sharing the same kqueue descriptor.
This vulnerability falls under the category of improper handling of concurrent processes and resource management within the kernel space. The technical implementation involves the kqueue system call mechanism which provides an efficient way for processes to monitor multiple file descriptors and events. When both parent and child processes attempt to register events for the same queue simultaneously, the kernel's internal data structures become corrupted or inconsistent, resulting in system crashes or unresponsive behavior. The issue demonstrates a classic race condition problem where multiple processes access shared kernel resources without proper synchronization mechanisms.
The operational impact of this vulnerability extends beyond simple denial of service to potentially compromise the entire system stability. Local attackers can exploit this weakness to crash the kernel, forcing system restarts and disrupting legitimate user activities. In environments where Mac OS X systems handle critical workloads or serve multiple users, such an exploit could lead to significant downtime and productivity loss. The vulnerability affects the core kernel functionality and can be triggered through relatively simple process forking operations, making it particularly dangerous as it requires minimal privileges and technical expertise to execute.
Mitigation strategies for this vulnerability should focus on both immediate system hardening and long-term architectural improvements. System administrators should apply the relevant security patches provided by Apple to address the kernel-level flaw directly. Additionally, process monitoring should be implemented to detect unusual kqueue usage patterns that might indicate exploitation attempts. The implementation of proper kernel-level synchronization mechanisms and enhanced resource validation would address the root cause. This vulnerability aligns with CWE-362, which describes concurrent execution use of lock, and relates to ATT&CK technique T1499.004, which covers network denial of service attacks through kernel manipulation. Organizations should also consider implementing process isolation techniques and monitoring for abnormal fork patterns that could indicate attempts to exploit this specific kernel weakness.