CVE-2016-6246 in OpenBSD
Summary
by MITRE
OpenBSD 5.8 and 5.9 allows certain local users with kern.usermount privileges to cause a denial of service (kernel panic) by mounting a tmpfs with a VNOVAL in the (1) username, (2) groupname, or (3) device name of the root node.
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Analysis
by VulDB Data Team • 09/04/2020
The vulnerability described in CVE-2016-6246 represents a critical kernel-level flaw in OpenBSD versions 5.8 and 5.9 that allows local users with specific privileges to trigger a system-wide kernel panic. This issue specifically targets the tmpfs filesystem implementation and exploits a weakness in how the kernel handles certain mount parameters, particularly those related to username, groupname, and device name attributes of the root node. The vulnerability operates through a carefully crafted tmpfs mount operation that includes a VNOVAL value, which is a special constant used in the kernel to indicate that a particular field should not be set or validated.
The technical flaw manifests when a user with kern.usermount privileges attempts to mount a tmpfs filesystem while specifying a VNOVAL value in any of the three mentioned attributes. This particular configuration causes the kernel to enter an invalid state during the mount operation, leading to an immediate kernel panic that results in a complete system crash. The vulnerability is particularly concerning because it operates at the kernel level and requires minimal privileges to exploit, making it accessible to local users who have already gained some level of system access. The root cause of this vulnerability aligns with CWE-122, which deals with buffer overflow conditions in kernel space, and more specifically with improper validation of input parameters during filesystem operations.
The operational impact of this vulnerability extends beyond simple denial of service, as it can lead to complete system unavailability and potential data loss. When a kernel panic occurs, the system becomes unresponsive and must be rebooted to restore normal operations, potentially disrupting services and causing downtime for critical infrastructure. The vulnerability affects systems running OpenBSD 5.8 and 5.9, which were widely deployed in server and workstation environments, making the potential impact substantial. From an attacker's perspective, this represents a reliable method for causing system instability, which could be leveraged as part of a broader attack strategy or as a means to escalate privileges through system disruption.
The exploitability of this vulnerability demonstrates a fundamental flaw in kernel input validation mechanisms and highlights the importance of proper parameter checking in system-level code. The fact that it requires only kern.usermount privileges indicates that even users with limited system access can potentially cause significant disruption. Mitigation strategies should focus on updating to patched versions of OpenBSD where this vulnerability has been resolved through proper kernel parameter validation and input sanitization. Organizations should also implement monitoring for unusual mount operations and consider restricting user mount privileges where possible. This vulnerability aligns with ATT&CK technique T1499.001, which involves network denial of service attacks, though in this case the attack operates at the local kernel level rather than through network protocols. The remediation process involves applying the appropriate OpenBSD security patches and ensuring that all systems running affected versions are updated to prevent exploitation.