CVE-2018-6559 in Linux
Summary
by MITRE
The Linux kernel, as used in Ubuntu 18.04 LTS and Ubuntu 18.10, allows local users to obtain names of files in which they would not normally be able to access via an overlayfs mount inside of a user namespace.
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Analysis
by VulDB Data Team • 06/03/2023
The vulnerability identified as CVE-2018-6559 represents a significant privilege escalation flaw within the Linux kernel's overlay filesystem implementation. This issue affects Ubuntu 18.04 LTS and Ubuntu 18.10 releases, where local users can exploit a design weakness in overlayfs to gain unauthorized access to files that should normally be restricted. The vulnerability specifically manifests when overlayfs is mounted within a user namespace, creating a path for information disclosure that bypasses standard access controls. The flaw stems from insufficient validation of file access permissions when overlay filesystems are used in conjunction with user namespaces, allowing malicious users to enumerate and potentially access files outside their designated access boundaries.
The technical root cause of this vulnerability lies in the improper handling of file permissions within the overlay filesystem's mount operation when executed in user namespace contexts. Overlayfs is designed to present multiple directories as a single unified filesystem, but the kernel implementation fails to properly enforce access controls when these mounts occur within user namespaces. This creates a scenario where the security boundaries established by user namespaces can be circumvented, enabling information disclosure through file name enumeration. The vulnerability is categorized under CWE-200 (Information Exposure) and specifically relates to improper access control mechanisms within the Linux kernel's filesystem layer.
The operational impact of CVE-2018-6559 extends beyond simple information disclosure, as it provides attackers with reconnaissance capabilities that can lead to more severe exploitation vectors. An attacker with local access can use this vulnerability to discover the existence of sensitive files and directories that would normally be hidden from them, potentially revealing system configuration details, user data, or application-specific information. This reconnaissance capability can serve as a foundation for subsequent attacks, including privilege escalation attempts or targeted exploitation of other vulnerabilities. The vulnerability affects systems running affected Ubuntu versions and can be exploited by any local user, making it particularly concerning for multi-user environments where isolation between users is expected. According to ATT&CK framework, this vulnerability maps to T1068 (Local Privilege Escalation) and T1083 (File and Directory Discovery) techniques, as it enables both information gathering and privilege escalation capabilities.
Mitigation strategies for CVE-2018-6559 focus primarily on system updates and kernel patches provided by Ubuntu. The most effective solution involves applying the official security patches released by Ubuntu for affected versions, which address the underlying permission checking mechanism in overlayfs. System administrators should also consider implementing additional security measures such as restricting user namespace capabilities, disabling overlayfs mounts when not required, and monitoring for suspicious filesystem activity. The vulnerability demonstrates the importance of proper namespace isolation and highlights the need for comprehensive access control validation even within seemingly secure kernel subsystems. Organizations should conduct thorough security assessments of their systems to identify any instances where overlayfs is being used in user namespace contexts and ensure that all systems are updated to patched kernel versions to prevent exploitation of this vulnerability.