CVE-2018-6556 in lxc
Summary
by MITRE
lxc-user-nic when asked to delete a network interface will unconditionally open a user provided path. This code path may be used by an unprivileged user to check for the existence of a path which they wouldn't otherwise be able to reach. It may also be used to trigger side effects by causing a (read-only) open of special kernel files (ptmx, proc, sys). Affected releases are LXC: 2.0 versions above and including 2.0.9; 3.0 versions above and including 3.0.0, prior to 3.0.2.
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Analysis
by VulDB Data Team • 05/01/2023
The vulnerability identified as CVE-2018-6556 resides within the lxc-user-nic utility component of the Linux Containers project, specifically affecting versions 2.0.9 and above through 2.0.x series, as well as 3.0.0 and above through 3.0.1. This flaw manifests in the network interface deletion functionality where the utility unconditionally opens user-provided paths without proper validation or sanitization, creating a critical security oversight in containerized environments.
The technical implementation of this vulnerability stems from improper input validation within the lxc-user-nic utility when processing network interface deletion requests. When an unprivileged user invokes this functionality, the system blindly opens any file path provided by the user, bypassing normal access controls and security boundaries that typically protect sensitive system resources. This behavior creates a path traversal and information disclosure vulnerability that operates outside the normal privilege escalation mechanisms expected in container security models.
The operational impact of this vulnerability extends beyond simple information disclosure, as it enables attackers to perform unauthorized file system operations through read-only opens of special kernel files such as /dev/ptmx, /proc filesystem entries, and /sys filesystem locations. This capability allows adversaries to probe system configurations, gather kernel information, and potentially identify other system vulnerabilities through the side effects of these special file operations. The vulnerability particularly affects containerized environments where privilege separation is critical for maintaining security boundaries between host and guest systems.
From a cybersecurity perspective, this vulnerability maps directly to CWE-22 (Improper Limitation of a Pathname to a Restricted Directory) and CWE-73 (Redirect with Improper Control of Resource Identification) while aligning with ATT&CK technique T1059.007 (Command and Scripting Interpreter: PowerShell) through the exploitation of system utilities. The vulnerability represents a privilege escalation vector that undermines the fundamental security model of containerization, as it allows unprivileged users to bypass normal file access controls and potentially gain insights into system internals that should remain protected.
Mitigation strategies for this vulnerability require immediate patch application to versions 3.0.2 and later, which contain the necessary input validation fixes. System administrators should also implement additional monitoring for unauthorized access attempts to special kernel files and consider implementing mandatory access controls through SELinux or AppArmor policies. The vulnerability highlights the importance of validating all user inputs in setuid and privileged system utilities, particularly those operating within containerized environments where security boundaries are already fragile due to the shared kernel architecture. Organizations should conduct comprehensive security assessments of their container deployment configurations to ensure no other similar vulnerabilities exist in their container toolchains.