CVE-2017-15131 in xdg-user-dirs
Summary
by MITRE
It was found that system umask policy is not being honored when creating XDG user directories, since Xsession sources xdg-user-dirs.sh before setting umask policy. This only affects xdg-user-dirs before 0.15.5 as shipped with Red Hat Enterprise Linux.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 12/20/2019
The vulnerability described in CVE-2017-15131 represents a critical access control issue within the XDG user directory creation mechanism on Linux systems. This flaw specifically impacts the enforcement of system-wide umask policies during the initialization of user desktop environments, creating potential security implications for file and directory permissions. The issue manifests when XDG user directories are created through the standard desktop session startup process, where the sequence of operations fails to properly apply the system's intended permission policies.
The technical root cause of this vulnerability lies in the improper ordering of operations within the Xsession initialization script. When a user logs into a desktop environment, the system typically sources the xdg-user-dirs.sh script before applying the system umask policy. This sequence means that user directories are created with default permissions that may not align with the organization's security requirements. The umask setting, which controls the default permissions for newly created files and directories, is effectively bypassed during the creation of XDG directories, potentially allowing unauthorized access to user data. This behavior creates a persistent security gap where sensitive user information could be exposed due to overly permissive default permissions.
The operational impact of this vulnerability extends beyond simple permission misconfigurations to potentially compromise user privacy and system integrity. When XDG user directories such as Documents, Downloads, or Pictures are created without proper umask enforcement, they may inherit permissions that are more open than intended. This situation can lead to information disclosure vulnerabilities where other users on the same system or even network-based attackers could access sensitive user files. The vulnerability affects systems running xdg-user-dirs versions prior to 0.15.5, which were distributed with Red Hat Enterprise Linux, making it particularly concerning for enterprise environments that rely on these distributions for their desktop infrastructure.
This vulnerability maps to CWE-276, which specifically addresses improper file permissions, and aligns with ATT&CK technique T1068, which involves privilege escalation through improper access control mechanisms. The flaw demonstrates how seemingly minor configuration issues in desktop environment initialization can create significant security weaknesses that persist throughout user sessions. Organizations implementing security controls should consider this vulnerability as part of their desktop environment hardening requirements, particularly in multi-user environments where proper access control is essential.
The recommended mitigation strategy involves updating the xdg-user-dirs package to version 0.15.5 or later, which contains the fix for this specific ordering issue. System administrators should also implement regular patch management processes to ensure that desktop environment components remain up to date with security fixes. Additional monitoring should be implemented to detect any unauthorized changes to user directory permissions that might indicate exploitation of this vulnerability. Organizations may also consider implementing automated compliance checks that verify proper umask enforcement during desktop session initialization, providing an additional layer of security assurance beyond simple patching.