CVE-2019-3689 in Linux Enterprise Server
Summary
by MITRE
The nfs-utils package in SUSE Linux Enterprise Server 12 before and including version 1.3.0-34.18.1 and in SUSE Linux Enterprise Server 15 before and including version 2.1.1-6.10.2 the directory /var/lib/nfs is owned by statd:nogroup. This directory contains files owned and managed by root. If statd is compromised, it can therefore trick processes running with root privileges into creating/overwriting files anywhere on the system if fs.protected_symlinks is not set
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Analysis
by VulDB Data Team • 12/26/2023
The vulnerability described in CVE-2019-3689 represents a critical privilege escalation risk within the NFS utilities implementation on SUSE Linux Enterprise Server systems. This issue stems from improper directory ownership and file management practices within the NFS service infrastructure, specifically affecting versions of nfs-utils prior to 1.3.0-34.18.1 on SLES 12 and 2.1.1-6.10.2 on SLES 15. The root cause lies in the misconfiguration where the /var/lib/nfs directory is owned by the statd user with the nogroup group, while the files within this directory are managed by the root user. This creates a dangerous privilege separation scenario where a compromised statd service could potentially manipulate file operations that would normally require root privileges, exploiting a fundamental flaw in the system's access control mechanisms.
The technical exploitation of this vulnerability occurs through a combination of file system permissions and symlink manipulation. When the statd service is compromised, it can leverage the directory ownership structure to create or overwrite files anywhere on the system through a carefully crafted symlink attack. This attack vector specifically requires that the kernel parameter fs.protected_symlinks is not set, which would otherwise prevent such privilege escalation through symbolic link traversal. The vulnerability aligns with CWE-276, which describes improper file permissions, and represents a classic case of privilege escalation through insecure file handling. The attack exploits the trust relationship between system services and file system operations, allowing a low-privilege compromised service to gain elevated privileges through manipulation of the file system's symbolic link resolution process.
The operational impact of this vulnerability extends beyond simple privilege escalation to potentially compromise the entire system integrity and availability. An attacker who gains control of the statd service can effectively take control of the NFS infrastructure and use this foothold to modify critical system files, install persistent backdoors, or escalate privileges to gain full root access. This vulnerability affects systems that rely heavily on NFS services for file sharing and storage operations, making it particularly dangerous in enterprise environments where NFS is commonly used for distributed storage solutions. The attack requires minimal privileges to initiate but can result in complete system compromise, making it a high-severity issue that impacts system availability, confidentiality, and integrity. This vulnerability also aligns with ATT&CK technique T1068, which covers "Exploitation for Privilege Escalation," and demonstrates how insecure file system permissions can be exploited to gain unauthorized access to system resources.
Mitigation strategies for this vulnerability should focus on immediate patching of affected nfs-utils versions, proper directory ownership configuration, and kernel parameter hardening. Systems should be updated to versions of nfs-utils that resolve the directory ownership issue and implement proper file system permissions. The fs.protected_symlinks kernel parameter should be enabled to prevent symlink-based privilege escalation attacks. Additionally, system administrators should implement proper monitoring of NFS service operations and file system changes within the /var/lib/nfs directory. Network segmentation and access controls should be implemented to limit exposure of NFS services to only trusted networks and hosts. The vulnerability highlights the importance of maintaining proper file system permissions and access controls as outlined in security best practices, particularly in enterprise environments where distributed file systems are commonly deployed. Organizations should also implement regular security audits to identify and remediate similar permission misconfigurations across their infrastructure.