CVE-2016-2853 in Linux
Summary
by MITRE
The aufs module for the Linux kernel 3.x and 4.x does not properly restrict the mount namespace, which allows local users to gain privileges by mounting an aufs filesystem on top of a FUSE filesystem, and then executing a crafted setuid program.
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Analysis
by VulDB Data Team • 11/01/2024
The vulnerability described in CVE-2016-2853 resides within the aufs (Advanced Multi-Layered Unification File System) module of the Linux kernel version 3.x and 4.x series. This flaw represents a significant privilege escalation vulnerability that exploits the improper handling of mount namespaces, creating a dangerous intersection between different filesystem types. The vulnerability specifically affects systems where aufs is enabled and allows local attackers to leverage the filesystem layering mechanism to bypass security controls that normally protect against privilege escalation attacks.
The technical flaw occurs when the aufs module fails to properly enforce namespace restrictions during mount operations. When a user mounts an aufs filesystem on top of a FUSE (Filesystem in Userspace) filesystem, the kernel's mount namespace handling becomes compromised. This creates a scenario where the security boundaries between different filesystem layers are not properly maintained, allowing for potential privilege escalation. The vulnerability is particularly dangerous because it exploits the interaction between two distinct filesystem mechanisms that should normally operate independently but become entangled through the flawed aufs implementation.
The operational impact of this vulnerability is severe for systems running affected kernel versions, as it enables local users to escalate their privileges from standard user level to root access. Attackers can exploit this by first mounting an aufs filesystem over a FUSE filesystem, then executing a specially crafted setuid program that takes advantage of the compromised namespace restrictions. This attack vector bypasses traditional privilege separation mechanisms and can be particularly devastating in multi-user environments or systems where users have the ability to create mount points. The vulnerability essentially undermines the kernel's security model by allowing unauthorized privilege elevation through legitimate filesystem operations.
The exploitability of this vulnerability aligns with several ATT&CK techniques including privilege escalation and defense evasion, specifically targeting the T1068 privilege escalation tactic and T1564.1001 hiding files and directories sub-technique. From a CWE perspective, this vulnerability maps to CWE-276 which describes improper privilege management, and CWE-264 which covers permissions, privileges, and access controls. The vulnerability demonstrates a classic case of insufficient restriction of operations within a namespace, where the mount namespace does not properly enforce the security boundaries that should prevent such cross-layer access. Organizations should immediately apply kernel updates to address this vulnerability and implement monitoring for unauthorized mount operations, particularly those involving aufs and FUSE filesystem combinations. System administrators should also consider restricting the ability to mount aufs filesystems in environments where this vulnerability could be exploited, and regularly audit filesystem configurations to ensure proper namespace isolation is maintained.