CVE-2016-1575 in Linux
Summary
by MITRE
The overlayfs implementation in the Linux kernel through 4.5.2 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 • 11/30/2024
The vulnerability identified as CVE-2016-1575 resides within the overlayfs filesystem implementation in Linux kernel versions through 4.5.2, representing a critical privilege escalation flaw that exploits improper handling of POSIX Access Control Lists. This issue specifically targets the management of extended attributes related to POSIX ACLs, creating a pathway for local attackers to elevate their privileges through carefully crafted filesystem operations. The flaw manifests when the overlayfs filesystem fails to correctly maintain ACL metadata during file operations, particularly affecting scenarios involving group-writable setgid directories that are commonly used in multi-user environments. The vulnerability operates at the kernel level, making it particularly dangerous as it bypasses traditional user-space security controls and directly exploits kernel filesystem mechanisms.
The technical root cause of this vulnerability stems from the overlayfs implementation's inadequate handling of extended attribute data when files are created or modified within overlay filesystems. When a file is created in an overlayfs mount, the system should properly preserve and propagate POSIX ACL information from the underlying filesystem layers. However, the kernel implementation fails to correctly manage the extended attributes associated with Access Control Lists, specifically the xattr data that stores ACL permissions. This failure becomes exploitable when a local user places a file in a group-writable setgid directory, allowing them to manipulate the ACL data in ways that should be restricted by normal filesystem permissions. The flaw creates a condition where the kernel's ACL handling logic does not properly validate or enforce access controls during overlay operations, leading to potential privilege escalation.
The operational impact of CVE-2016-1575 extends beyond simple privilege escalation, as it can enable attackers to gain unauthorized access to sensitive system resources and potentially compromise entire system integrity. Local users who can write to group-writable setgid directories can exploit this vulnerability to modify file permissions and ACLs in ways that would normally be restricted. This creates opportunities for attackers to establish persistent access, escalate privileges to root level, and potentially access confidential data or system resources that should be protected. The vulnerability is particularly concerning in multi-user environments where setgid directories are commonly used for shared access control, as it can allow attackers to bypass normal security boundaries and gain elevated privileges without requiring physical access or additional attack vectors. The impact is further amplified because the vulnerability exists in kernel code, meaning that successful exploitation can result in complete system compromise.
Mitigation strategies for CVE-2016-1575 focus primarily on kernel updates and system hardening measures that address the underlying filesystem implementation flaw. The most effective solution involves upgrading to a Linux kernel version that includes the patch for this vulnerability, typically kernel versions 4.6 and later where the overlayfs implementation was corrected to properly handle POSIX ACL extended attributes. System administrators should also implement additional security measures such as restricting access to group-writable setgid directories, monitoring for unusual file creation patterns, and ensuring that filesystem permissions are properly configured to minimize the attack surface. The vulnerability aligns with CWE-276, which describes improper file permissions, and relates to ATT&CK technique T1068, privilege escalation through local exploits. Organizations should also consider implementing filesystem monitoring tools to detect potential exploitation attempts and maintain regular kernel updates to protect against similar vulnerabilities in the future.