CVE-2016-2854 in Linux
Summary
by MITRE
The aufs module for the Linux kernel 3.x and 4.x does not properly maintain POSIX ACL xattr data, which allows local users to gain privileges by leveraging a group-writable setgid directory.
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Analysis
by VulDB Data Team • 10/31/2024
The vulnerability identified as CVE-2016-2854 affects the aufs (Advanced Multi-Layered Unification File System) module within Linux kernel versions 3.x and 4.x, representing a critical security flaw that undermines the fundamental access control mechanisms of the operating system. This issue specifically targets the improper handling of POSIX Access Control Lists (ACLs) through extended attributes, creating a privilege escalation vector that can be exploited by local attackers. The vulnerability stems from the kernel's failure to correctly maintain the relationship between extended attributes and the underlying file permissions, particularly when dealing with setgid directories that are writable by groups.
The technical flaw manifests when the aufs module processes extended attributes associated with POSIX ACLs, failing to properly validate or preserve the access control information during file system operations. This mismanagement occurs specifically in scenarios involving group-writable setgid directories where the module does not correctly enforce the expected POSIX ACL behavior. The vulnerability is classified under CWE-276, which addresses improper privilege management, and represents a classic case of inadequate access control validation. When a local user operates within a group-writable setgid directory, the flawed aufs implementation allows the attacker to manipulate extended attributes in a manner that bypasses normal permission checks, ultimately enabling privilege escalation to root level access.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it provides attackers with a reliable method to gain unauthorized administrative access to affected systems. The exploitability of CVE-2016-2854 is particularly concerning because it requires minimal prerequisites - essentially any local user with access to a group-writable setgid directory can leverage this flaw. This creates a significant risk for systems where such directories exist, which are common in multi-user environments, shared workspaces, or development systems. The vulnerability aligns with ATT&CK technique T1068, which covers 'Local Privilege Escalation,' and specifically demonstrates how file system implementation flaws can be exploited to bypass security controls. Systems running kernel versions 3.x and 4.x that utilize aufs as their file system layer are particularly at risk, making this vulnerability a critical concern for enterprise environments and cloud deployments that rely on these kernel versions.
Mitigation strategies for CVE-2016-2854 primarily involve upgrading to patched kernel versions where the aufs module properly handles POSIX ACL extended attributes. System administrators should prioritize kernel updates that address this specific vulnerability, as there are no effective workarounds that can be implemented without modifying the underlying file system behavior. Organizations should also conduct thorough inventory assessments to identify systems running affected kernel versions and those utilizing aufs file systems. Additional defensive measures include implementing strict directory permissions policies to minimize the presence of group-writable setgid directories, monitoring for unauthorized access to sensitive system areas, and ensuring comprehensive patch management processes are in place to address similar vulnerabilities. The vulnerability highlights the importance of maintaining up-to-date kernel versions and proper file system implementation validation, particularly for critical system components like aufs that handle core access control functions.