CVE-2016-1237 in Linux
Summary
by MITRE
nfsd in the Linux kernel through 4.6.3 allows local users to bypass intended file-permission restrictions by setting a POSIX ACL, related to nfs2acl.c, nfs3acl.c, and nfs4acl.c.
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Analysis
by VulDB Data Team • 08/25/2022
The vulnerability identified as CVE-2016-1237 represents a critical access control flaw within the Linux kernel's Network File System daemon implementation. This issue affects kernel versions through 4.6.3 and specifically targets the nfsd component responsible for handling network file system operations. The flaw enables local attackers to circumvent intended file permission restrictions through the manipulation of POSIX Access Control Lists, creating a significant security breach in network file sharing environments. The vulnerability stems from inadequate validation of access control mechanisms within the NFS implementation, particularly when processing POSIX ACLs for different NFS protocol versions.
The technical implementation of this vulnerability resides in the nfs2acl.c, nfs3acl.c, and nfs4acl.c source files that handle the translation and enforcement of access control lists between NFS protocol versions and the local filesystem. These modules fail to properly validate or enforce the access control restrictions when POSIX ACLs are applied to NFS shared files, allowing local users to escalate their privileges beyond what the intended file permissions would permit. The flaw occurs during the processing of access control list modifications, where the kernel does not adequately verify that the requesting user has sufficient permissions to modify the ACL settings, particularly when transitioning between different NFS protocol versions that have varying access control semantics.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it fundamentally undermines the security model of NFS file sharing systems. Local users can exploit this flaw to gain unauthorized access to files and directories that should be restricted, potentially leading to data leakage, modification of sensitive files, or complete compromise of the file system. This vulnerability is particularly dangerous in multi-user environments where NFS is extensively used for shared storage, as it allows attackers to bypass the security controls that are meant to protect against unauthorized access. The implications are severe for enterprise environments that rely on NFS for file sharing, as this flaw could enable attackers to access confidential data or disrupt file system operations.
Security mitigations for this vulnerability require immediate kernel updates to versions that contain the patched implementation of the NFS ACL handling code. System administrators should prioritize applying the relevant security patches from their respective Linux distributions, as the vulnerability affects core kernel functionality that is critical to network file sharing operations. Additionally, organizations should review their NFS configurations and access controls to ensure that unnecessary local access is minimized, and implement monitoring for unauthorized ACL modifications. This vulnerability aligns with CWE-284, which describes improper access control issues, and represents a specific instance of privilege escalation through flawed access control enforcement. The flaw also maps to ATT&CK technique T1068, which covers local privilege escalation, as attackers can leverage this vulnerability to gain elevated system privileges through local access. Organizations should conduct comprehensive security assessments of their NFS implementations to identify systems running vulnerable kernel versions and implement proper access controls to minimize the attack surface.