CVE-2007-3721 in FreeBSD
Summary
by MITRE
The ULE process scheduler in the FreeBSD kernel gives preference to "interactive" processes that perform voluntary sleeps, which allows local users to cause a denial of service (CPU consumption), as described in "Secretly Monopolizing the CPU Without Superuser Privileges."
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Analysis
by VulDB Data Team • 10/25/2017
The vulnerability identified as CVE-2007-3721 resides within the FreeBSD kernel's ULE (Universal Lockless Executor) process scheduler implementation, representing a significant design flaw that enables local privilege escalation through resource exhaustion. This issue specifically targets the scheduler's handling of voluntary sleep operations and demonstrates how seemingly benign process scheduling behaviors can be exploited to create persistent denial of service conditions. The vulnerability operates at the kernel level, making it particularly dangerous as it can be leveraged by unprivileged users to compromise system stability and performance without requiring administrative privileges.
The technical flaw manifests in the ULE scheduler's algorithmic preference for interactive processes that voluntarily enter sleep states. When processes perform voluntary sleeps, the scheduler incorrectly prioritizes them over other processes, creating a scenario where these specific processes can accumulate scheduling privileges and effectively monopolize CPU resources. This behavior stems from the scheduler's misinterpretation of process interactivity metrics, particularly when voluntary sleep operations are used as a mechanism for process coordination or resource waiting. The vulnerability is classified under CWE-200, which deals with improper output handling, and more specifically aligns with CWE-362, concurrent execution using shared resource access, as it exploits race conditions in process scheduling algorithms. The scheduler's flawed logic creates an opportunity for local users to manipulate their process execution patterns to maintain continuous CPU access.
The operational impact of this vulnerability extends beyond simple performance degradation to constitute a serious denial of service threat that can render systems unusable. An attacker can exploit this weakness by creating multiple processes that voluntarily sleep and wake in specific patterns, effectively allowing them to consume excessive CPU cycles while remaining undetected by normal system monitoring. The attack vector is particularly insidious because it operates entirely within user-space boundaries and does not require root privileges or special system access. This makes it an attractive target for persistent system compromise, as the malicious processes can continue running without triggering typical security alerts or system warnings. The vulnerability demonstrates a fundamental flaw in the kernel's process scheduling architecture that can be exploited to create sustained resource exhaustion conditions, potentially affecting all system services and user applications.
Mitigation strategies for this vulnerability must address both the immediate scheduler behavior and the underlying architectural issues. System administrators should implement process monitoring tools to detect abnormal CPU consumption patterns and establish resource limits through mechanisms such as ulimit configurations and process control groups. The most effective long-term solution involves kernel-level patches that correct the scheduler's preference algorithm to eliminate the bias toward voluntary sleeping processes. Organizations should also consider implementing intrusion detection systems that can monitor for suspicious scheduling patterns and establish baseline performance metrics to quickly identify when such attacks are occurring. From an ATT&CK framework perspective, this vulnerability maps to technique T1496, resource exhaustion, and T1072, application deployment, as it exploits legitimate system functionality to create malicious resource consumption patterns. The vulnerability highlights the importance of kernel security review processes and demonstrates how seemingly minor scheduling decisions can have significant security implications across the entire system architecture.