CVE-2016-6243 in OpenBSD
Summary
by MITRE
thrsleep in kern/kern_synch.c in OpenBSD 5.8 and 5.9 allows local users to cause a denial of service (kernel panic) via a crafted value in the tsp parameter of the __thrsleep system call.
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Analysis
by VulDB Data Team • 09/04/2020
The vulnerability identified as CVE-2016-6243 represents a critical kernel-level flaw in OpenBSD versions 5.8 and 5.9 that manifests through the thrsleep function located in kern/kern_synch.c. This issue constitutes a local privilege escalation vulnerability that can be exploited by malicious users with access to the system to trigger a kernel panic, effectively causing a denial of service condition that renders the entire system unstable and unresponsive. The vulnerability specifically targets the __thrsleep system call which is responsible for thread suspension and synchronization mechanisms within the operating system's kernel. When a crafted value is provided in the tsp parameter of this system call, the kernel's thread management subsystem fails to properly validate the input, leading to unpredictable behavior that ultimately results in system crash. This flaw operates at the kernel level, making it particularly dangerous as it bypasses normal user-space security mechanisms and directly impacts the core operating system functionality. The vulnerability stems from inadequate input validation within the thread sleep synchronization logic, where the kernel fails to properly sanitize or verify the timestamp parameter that controls thread suspension behavior.
The technical implementation of this vulnerability demonstrates a classic case of improper input validation that can be classified under CWE-20, which specifically addresses "Improper Input Validation" in software systems. The flaw occurs when the thrsleep function processes a maliciously crafted tsp parameter that contains invalid or unexpected values, causing the kernel to attempt operations on corrupted data structures or memory locations. This type of vulnerability falls under the ATT&CK technique T1068, which involves the exploitation of legitimate credentials and system access to gain elevated privileges and compromise system stability. The kernel panic occurs because the invalid timestamp parameter causes the thread scheduler to enter an inconsistent state where it attempts to access memory locations that have been corrupted or improperly initialized. The system call interface for __thrsleep is designed to handle thread suspension with timeout functionality, but the lack of proper bounds checking on the tsp parameter allows attackers to manipulate the internal thread management structures in ways that were not anticipated during the kernel's design phase.
From an operational perspective, this vulnerability creates significant risks for systems running affected OpenBSD versions as it allows local users to induce system-wide crashes that can result in complete service disruption. The impact extends beyond simple denial of service since a kernel panic can potentially cause data loss, system corruption, and require extensive manual intervention to restore normal operations. The vulnerability is particularly concerning because it requires no special privileges beyond local access, meaning any user with shell access to the system can potentially exploit it. Attackers could leverage this vulnerability to repeatedly crash the system, creating persistent availability issues, or to obscure other malicious activities by causing system instability. The flaw affects the fundamental thread synchronization mechanisms that underpin many system services, making it difficult to predict exactly which processes might be impacted or how the system behavior might change during exploitation. Additionally, the vulnerability could be chained with other exploits to create more sophisticated attack vectors, as the kernel panic state might provide opportunities for privilege escalation or information disclosure.
Mitigation strategies for CVE-2016-6243 should focus on immediate system updates to patched versions of OpenBSD that address the improper input validation in the thrsleep function. System administrators should prioritize applying security patches from OpenBSD's official release notes and security advisories, as these updates contain the necessary code modifications to properly validate the tsp parameter before processing. The recommended approach involves upgrading to OpenBSD versions 5.10 or later, where the vulnerability has been resolved through improved input validation mechanisms in the kernel's thread management subsystem. Organizations should also implement monitoring systems to detect unusual patterns of system crashes or kernel panics that might indicate exploitation attempts, as these events can serve as early warning signs of potential attacks. Additional defensive measures include implementing proper access controls to limit local user privileges, regularly reviewing system logs for suspicious activities, and maintaining comprehensive backup and recovery procedures to quickly restore services in case of exploitation. The vulnerability underscores the importance of maintaining up-to-date system patches and proper kernel security practices, as it represents a failure in the kernel's input validation that could have been prevented through proper code review and security testing procedures.