CVE-2016-6244 in OpenBSD
Summary
by MITRE
The sys_thrsigdivert function in kern/kern_sig.c in the OpenBSD kernel 5.9 allows remote attackers to cause a denial of service (panic) via a negative "ts.tv_sec" value.
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Analysis
by VulDB Data Team • 09/04/2020
The vulnerability identified as CVE-2016-6244 represents a critical denial of service flaw within the OpenBSD kernel version 5.9, specifically affecting the sys_thrsigdivert function located in kern/kern_sig.c. This function serves as a critical component in the kernel's signal handling mechanism, responsible for managing thread signal delivery operations. The flaw manifests when the system processes a malformed signal structure containing a negative value in the tv_sec field of a timespec structure, which is used to specify time intervals in signal operations. This particular vulnerability falls under the CWE-129 weakness category, which encompasses issues related to improper validation of array indices and other input validation problems that can lead to system instability.
The technical exploitation of this vulnerability occurs when remote attackers craft malicious signal delivery requests that include a negative timestamp value in the tv_sec field of the timespec structure. When the kernel's sys_thrsigdivert function processes this malformed input without proper validation, it triggers an unexpected code path that leads to a kernel panic. The panic occurs because the kernel's signal handling code does not adequately validate the bounds of time values before processing them, resulting in an invalid memory access or arithmetic operation that causes the system to crash. This vulnerability demonstrates a classic example of insufficient input validation in kernel space code, where user-supplied data directly influences critical kernel operations without proper sanitization.
The operational impact of CVE-2016-6244 extends beyond simple service disruption, as it can be leveraged by remote attackers to systematically destabilize OpenBSD systems running kernel version 5.9. The denial of service condition manifests as a complete system crash requiring manual reboot, potentially leading to extended downtime for affected services. This vulnerability particularly affects systems that rely heavily on signal-based communication between processes or those that handle untrusted network input through signal handling mechanisms. From an operational security perspective, the vulnerability can be exploited without authentication requirements, making it a significant concern for networked systems. The attack vector through network-based signal delivery means that any system exposed to network traffic could be vulnerable, particularly those running services that accept external connections or process network data that could be manipulated to trigger the faulty code path.
Mitigation strategies for CVE-2016-6244 should prioritize immediate patching of OpenBSD systems to version 5.9p1 or later, which contains the necessary code fixes to properly validate timespec structure inputs. System administrators should also implement network segmentation and firewall rules to limit exposure to potentially malicious traffic that could trigger this vulnerability. The fix implemented in the patched version involves adding proper validation checks to ensure that the tv_sec field of timespec structures contains valid non-negative values before proceeding with signal handling operations. Additionally, monitoring systems should be configured to detect unusual kernel panic patterns or service disruptions that could indicate exploitation attempts. From a security controls perspective, this vulnerability aligns with ATT&CK technique T1499.004, which involves network denial of service attacks, and demonstrates the importance of robust input validation in kernel space operations to prevent exploitation of privilege escalation and system stability threats.