CVE-2015-4176 in Linux
Summary
by MITRE
fs/namespace.c in the Linux kernel before 4.0.2 does not properly support mount connectivity, which allows local users to read arbitrary files by leveraging user-namespace root access for deletion of a file or directory.
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Analysis
by VulDB Data Team • 07/27/2022
The vulnerability described in CVE-2015-4176 resides within the Linux kernel's filesystem namespace implementation, specifically in the fs/namespace.c file. This flaw represents a critical security issue that affects Linux kernel versions prior to 4.0.2, where the kernel fails to properly maintain mount connectivity between different namespace contexts. The vulnerability stems from insufficient validation mechanisms that govern how mount points behave when transitioning between user namespaces, creating a pathway for privilege escalation and unauthorized data access.
The technical exploitation of this vulnerability relies on the specific interaction between user namespaces and mount operations within the Linux kernel's namespace subsystem. When a user namespace root gains access to a system, they can manipulate mount points in ways that bypass normal filesystem access controls. The flaw occurs because the kernel does not adequately verify mount connectivity when user namespaces are involved, allowing malicious actors to traverse filesystem boundaries and access files that should otherwise be restricted. This issue is particularly dangerous because it leverages legitimate kernel functionality to create unauthorized access paths, making detection more challenging.
From an operational impact perspective, this vulnerability enables local users with access to a user namespace to escalate privileges and read arbitrary files on the system. The attack vector requires the attacker to first obtain access to a user namespace, typically through privilege escalation or by exploiting another vulnerability, then leverage the mount connectivity flaw to access files outside their intended scope. This can result in information disclosure, potential system compromise, and unauthorized data access that could expose sensitive system information, configuration files, or user data. The vulnerability affects the fundamental security model of Linux kernel namespaces and undermines the isolation guarantees provided by user namespaces.
The security implications extend beyond simple file access, as this vulnerability can be chained with other exploits to create more sophisticated attack scenarios. According to CWE classification, this vulnerability maps to CWE-284 Access Control, specifically related to improper access control in namespace management. The ATT&CK framework would categorize this under privilege escalation techniques, specifically using kernel vulnerabilities to gain elevated privileges. Organizations should implement immediate kernel updates to version 4.0.2 or later to address this vulnerability, while also monitoring for potential exploitation attempts. Additional mitigations include restricting user namespace creation where possible, implementing proper access controls, and maintaining comprehensive system monitoring to detect unauthorized file access patterns that might indicate exploitation attempts.