CVE-2016-6242 in OpenBSD
Summary
by MITRE
OpenBSD 5.8 and 5.9 allows local users to cause a denial of service (assertion failure and kernel panic) via a large ident value in a kevent system call.
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Analysis
by VulDB Data Team • 09/04/2020
The vulnerability identified as CVE-2016-6242 represents a critical local privilege escalation issue affecting OpenBSD versions 5.8 and 5.9, specifically within the kernel's event notification mechanism. This flaw manifests through the kevent system call which is part of the kqueue framework used for asynchronous I/O operations and event monitoring. The vulnerability arises from insufficient input validation when processing ident values, which are used to identify events in the kernel's event queue system. When a malicious local user provides an excessively large ident value to the kevent system call, the kernel fails to properly handle this malformed input, leading to an assertion failure that ultimately results in a complete kernel panic and system crash.
The technical implementation of this vulnerability stems from a lack of proper bounds checking in the kernel's event handling code. The ident parameter in the kevent structure serves as a unique identifier for events, but the kernel's implementation fails to validate the maximum allowable size of this identifier. This validation gap creates an exploitable condition where an attacker can craft a malformed kevent request with an oversized ident value that exceeds the kernel's expected buffer boundaries. The assertion failure occurs during kernel execution when the system attempts to process the malformed event, triggering a cascade of failures that culminates in a kernel panic. This type of vulnerability falls under CWE-129, which specifically addresses insufficient input validation, and represents a classic example of an unchecked buffer access vulnerability in kernel space.
From an operational impact perspective, this vulnerability presents a significant threat to system availability and stability within OpenBSD environments. Since the vulnerability requires local user access to exploit, it primarily affects systems where local privilege escalation is possible or where users have the ability to execute processes that can make system calls. The denial of service impact is severe as a kernel panic causes immediate system shutdown and requires manual intervention to restore normal operation. The vulnerability's exploitation does not require elevated privileges to trigger the kernel panic, making it particularly dangerous in multi-user environments where local access might be more readily available. This vulnerability directly impacts the system's reliability and can be used as a vector for persistent denial of service attacks against critical infrastructure components running these vulnerable OpenBSD versions.
The mitigation strategy for CVE-2016-6242 involves immediate patching of affected OpenBSD systems to version 5.10 or later, which contains the necessary fixes for the input validation issue. System administrators should prioritize updating their OpenBSD installations to prevent exploitation, as the vulnerability can be reliably triggered by any local user with basic system access. Additionally, monitoring for anomalous system behavior or unexpected kernel panics should be implemented as part of defensive measures. Organizations running vulnerable systems should consider implementing process monitoring to detect potential exploitation attempts through malformed kevent system calls. The fix implemented in patched versions typically involves adding proper bounds checking to ensure ident values remain within acceptable ranges before processing, preventing the assertion failure that leads to kernel panic. This remediation aligns with ATT&CK technique T1068, which describes the use of local privilege escalation techniques, and demonstrates the importance of proper input validation in kernel space operations.