CVE-2011-4028 in X Server
Summary
by MITRE
The LockServer function in os/utils.c in X.Org xserver before 1.11.2 allows local users to determine the existence of arbitrary files via a symlink attack on a temporary lock file, which is handled differently if the file exists.
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Analysis
by VulDB Data Team • 12/05/2021
The vulnerability identified as CVE-2011-4028 resides within the X.Org xserver software, specifically in the LockServer function located in os/utils.c. This flaw represents a classic race condition scenario that enables local attackers to exploit symbolic link attacks against temporary lock files. The vulnerability affects versions of X.Org xserver prior to 1.11.2, making it a significant security concern for systems running older versions of this critical graphics server component. The issue stems from improper handling of temporary files during the locking process, creating an exploitable condition where malicious users can manipulate file system state to gain information about arbitrary files on the system.
The technical implementation of this vulnerability exploits a fundamental flaw in how the LockServer function manages temporary lock files. When the xserver attempts to create a lock file, it does not properly validate the existence of the target file before proceeding with operations. This creates a window where a local attacker can create a symbolic link pointing to a target file, and if the lock server's file operations are executed in a specific sequence, the symbolic link can be followed to reveal information about the target file. The vulnerability manifests because the system handles the lock file differently depending on whether it exists or not, creating a predictable information disclosure channel. This behavior directly relates to CWE-367, which addresses time-of-check to time-of-use (TOCTOU) race conditions, and represents a clear example of improper file system permission handling.
The operational impact of CVE-2011-4028 extends beyond simple information disclosure, as it provides attackers with a method to enumerate files on the system that they might not otherwise have access to. Local users can leverage this vulnerability to determine the existence of files in locations where they lack direct read permissions, potentially exposing sensitive system files, configuration data, or user information. The vulnerability is particularly concerning because it operates at the system level where the xserver runs with elevated privileges, allowing attackers to gather intelligence that could aid in further exploitation attempts. This type of vulnerability aligns with ATT&CK technique T1083, which covers file and directory discovery, and demonstrates how seemingly benign system utilities can be weaponized for reconnaissance purposes. The attack vector requires local system access but does not need additional privileges, making it particularly dangerous in multi-user environments where attackers might have limited access to system resources.
Mitigation strategies for CVE-2011-4028 focus primarily on updating to patched versions of the X.Org xserver, specifically versions 1.11.2 and later where the vulnerability has been addressed through proper file handling mechanisms. System administrators should prioritize patching affected systems, particularly those running older versions of X.Org xserver that remain in production environments. Additional protective measures include implementing proper file system permissions and ensuring that temporary directories used by the xserver have restricted access permissions to prevent symlink attacks. The vulnerability highlights the importance of proper race condition handling in system-level code and demonstrates why security-conscious development practices, including input validation and proper file access controls, are critical. Organizations should also consider implementing monitoring for unusual file system access patterns and maintain up-to-date vulnerability assessments to identify similar issues in other system components. The fix implemented in version 1.11.2 typically involves strengthening the file creation process to prevent symbolic link following during lock file operations, ensuring that file system operations are atomic and that proper validation occurs before any file system modifications take place.