CVE-2016-6247 in OpenBSD
Summary
by MITRE
OpenBSD 5.8 and 5.9 allows certain local users to cause a denial of service (kernel panic) by unmounting a filesystem with an open vnode on the mnt_vnodelist.
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Analysis
by VulDB Data Team • 09/04/2020
The vulnerability identified as CVE-2016-6247 represents a critical kernel-level flaw in OpenBSD versions 5.8 and 5.9 that enables local attackers to trigger a system-wide kernel panic through improper filesystem unmount operations. This issue resides within the kernel's handling of filesystem structures and demonstrates a fundamental flaw in the virtual node management system that governs how filesystems interact with kernel memory structures. The vulnerability specifically manifests when a filesystem is unmounted while having open virtual nodes associated with it, creating a race condition or memory management error that results in system instability and complete service disruption.
The technical root cause of this vulnerability stems from inadequate synchronization mechanisms and improper reference counting within the kernel's filesystem subsystem. When a filesystem is unmounted, the kernel must properly handle all active virtual nodes that reference the filesystem's data structures. In affected OpenBSD versions, the code path fails to adequately check for open vnodes before proceeding with the unmount operation, leading to dangling references and memory corruption. This type of vulnerability falls under CWE-119, which addresses improper access to memory locations, and represents a classic case of improper resource management in kernel space. The flaw operates at the intersection of filesystem management and kernel memory management, where the lack of proper validation leads to a kernel panic condition that terminates all system operations.
The operational impact of this vulnerability extends beyond simple denial of service to represent a serious threat to system availability and stability. Local users who can execute the specific sequence of operations can effectively crash the entire system, rendering it unusable until manual reboot occurs. This makes the vulnerability particularly dangerous in multi-user environments or systems where local access cannot be fully trusted. The attack vector requires local privileges but does not need elevated permissions, making it accessible to users who may not have administrative rights. From an adversarial perspective, this vulnerability aligns with ATT&CK technique T1499.004, which covers network denial of service attacks, though in this case the attack occurs at the kernel level rather than network infrastructure. The vulnerability also demonstrates characteristics of privilege escalation through system instability, as successful exploitation can lead to complete system compromise.
Mitigation strategies for CVE-2016-6247 focus primarily on upgrading to patched versions of OpenBSD where the kernel's filesystem unmount logic has been corrected. System administrators should immediately apply the relevant security patches provided by OpenBSD developers, which typically involve implementing proper reference counting and synchronization mechanisms during unmount operations. Additionally, monitoring systems should be configured to detect unusual filesystem unmount patterns, though this is more of a detection measure rather than a direct fix. Organizations should also consider implementing privilege separation techniques to limit local user access to filesystem operations that could trigger such conditions. The vulnerability highlights the importance of proper kernel memory management and synchronization primitives, reinforcing best practices from the CERT/CC guidelines for secure kernel development. Regular system updates and vulnerability assessments remain crucial for maintaining system integrity, as this type of kernel-level flaw represents a persistent threat to system availability that can be exploited by any local user with sufficient privileges to manipulate filesystem operations.