CVE-2005-4351 in FreeBSD
Summary
by MITRE
The securelevels implementation in FreeBSD 7.0 and earlier, OpenBSD up to 3.8, DragonFly up to 1.2, and Linux up to 2.6.15 allows root users to bypass immutable settings for files by mounting another filesystem that masks the immutable files while the system is running.
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Analysis
by VulDB Data Team • 08/01/2017
The vulnerability described in CVE-2005-4351 represents a critical flaw in the implementation of securelevels across multiple operating systems including FreeBSD, OpenBSD, DragonFly, and Linux. This issue fundamentally undermines the integrity of file system protection mechanisms that are designed to prevent unauthorized modifications to critical system files. The vulnerability specifically affects systems where securelevel functionality is implemented to enforce immutable file attributes, which are essential for maintaining system security and preventing malicious tampering with core system components.
The technical flaw lies in how these operating systems handle file system mounting operations when securelevels are active. When a root user mounts another filesystem on top of existing directories containing immutable files, the mounted filesystem effectively masks or shadows the original files, creating a situation where the immutable protection is bypassed. This occurs because the system's file access mechanisms do not properly account for the presence of mounted filesystems when enforcing securelevel restrictions. The underlying issue stems from improper handling of file system layering and access control enforcement during runtime operations, particularly when multiple filesystems are active simultaneously.
The operational impact of this vulnerability is severe and far-reaching for system administrators and security professionals. Root users can exploit this weakness to circumvent file protection mechanisms that are fundamental to system security, potentially allowing for persistent backdoor installations or system compromise without detection. This vulnerability essentially allows attackers with root privileges to bypass critical security controls that are meant to prevent unauthorized modifications to system files, effectively neutralizing the purpose of immutable file attributes. The attack vector is particularly concerning because it requires only root access, which is often considered the highest privilege level in Unix-like systems, making it a significant escalation point for malicious actors.
The vulnerability manifests through the specific implementation patterns found in the kernel code of affected operating systems, where the securelevel enforcement logic does not properly consider the implications of filesystem mounting operations. This flaw aligns with CWE-284 Access Control Issues, specifically related to insufficient enforcement of access control mechanisms during dynamic filesystem operations. From an ATT&CK framework perspective, this vulnerability maps to privilege escalation techniques and persistence mechanisms, as it allows for the establishment of persistent access to protected system resources. The vulnerability also intersects with defense evasion tactics, as it enables attackers to bypass security controls that are typically considered robust protection mechanisms.
Mitigation strategies for this vulnerability require careful system administration practices and kernel-level updates. System administrators should ensure that all affected operating systems are updated to versions that contain proper securelevel implementation fixes, which typically involve enhanced filesystem layering checks during mounting operations. Additionally, organizations should implement strict access control policies that limit root user activities and monitor filesystem mounting operations for suspicious patterns. The recommended approach includes disabling unnecessary filesystem mounting capabilities for root users, implementing proper audit logging of file system operations, and maintaining regular security updates to address known vulnerabilities in kernel components that handle securelevel enforcement.