CVE-2002-0820 in FreeBSD
Summary
by MITRE
FreeBSD kernel 4.6 and earlier closes the file descriptors 0, 1, and 2 after they have already been assigned to /dev/null when the descriptors reference procfs or linprocfs, which could allow local users to reuse the file descriptors in a setuid or setgid program to modify critical data and gain privileges.
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Analysis
by VulDB Data Team • 09/06/2019
The vulnerability identified as CVE-2002-0820 represents a critical flaw in the FreeBSD kernel version 4.6 and earlier systems that affects how file descriptor management operates within the operating system. This issue specifically targets the kernel's handling of standard input, output, and error file descriptors which are traditionally assigned to /dev/null during system initialization. The flaw occurs when these descriptors reference procfs or linprocfs filesystems, creating a scenario where the kernel prematurely closes these essential file descriptors before they are properly reassigned. This behavior creates a window of opportunity for local attackers to exploit the system's resource management mechanisms. The vulnerability stems from improper file descriptor lifecycle management within the kernel's process initialization routines, where the kernel fails to maintain proper reference counting or assignment protocols when dealing with virtual filesystems like procfs and linprocfs.
The technical implementation of this vulnerability exploits a race condition or improper resource handling in the kernel's file descriptor management subsystem. When the FreeBSD kernel initializes processes, it typically assigns file descriptors 0, 1, and 2 to /dev/null to ensure that all processes have standard input, output, and error streams available. However, in systems running kernel versions 4.6 and earlier, when these descriptors reference procfs or linprocfs, the kernel performs an early closure of these descriptors before they are properly reassigned. This premature closure leaves the file descriptors in a state where they can be reused by subsequent operations within the same process or by child processes. The flaw becomes particularly dangerous when exploited within setuid or setgid programs, as these programs typically run with elevated privileges and may have access to critical system resources that could be modified through the reused file descriptors.
The operational impact of this vulnerability is significant for FreeBSD systems running affected kernel versions, as it provides local attackers with a potential path to privilege escalation and system compromise. Attackers can leverage this vulnerability to manipulate critical system data by reusing the file descriptors that should remain closed or properly assigned to /dev/null. When exploited within setuid or setgid programs, the vulnerability allows attackers to potentially modify system files, configuration data, or other critical resources that these programs normally have access to. The implications extend beyond simple privilege escalation to include potential data corruption, system instability, and unauthorized access to sensitive information. This vulnerability directly relates to CWE-362, which describes concurrent execution using shared resource vulnerabilities, and may also align with CWE-242, concerning the use of potentially dangerous functions that could lead to privilege escalation through improper file descriptor handling. The attack vector primarily operates through local exploitation within the target system, making it particularly concerning for multi-user environments where privilege separation is critical.
The mitigation strategies for CVE-2002-0820 focus on immediate system updates and kernel version upgrades to address the underlying file descriptor management flaw. System administrators should prioritize upgrading to FreeBSD kernel versions 4.7 or later, where the vulnerability has been resolved through proper file descriptor lifecycle management. Additionally, implementing strict process isolation and privilege separation mechanisms can help reduce the attack surface, particularly for setuid and setgid programs that are most vulnerable to this exploitation. The recommended approach includes regular security patching, monitoring for unauthorized access attempts, and implementing proper file descriptor management practices within applications. Organizations should also consider implementing access control lists and privilege limiting mechanisms to prevent unauthorized modification of critical system resources. The vulnerability demonstrates the importance of proper resource management in kernel-level operations and highlights the necessity of thorough testing for race conditions and resource lifecycle management in operating system components. This issue also aligns with ATT&CK technique T1068, which covers the exploitation of system privileges through improper resource handling, emphasizing the need for robust kernel security controls and proper privilege management in operating system design.