CVE-2011-4105 in LightDM
Summary
by MITRE
LightDM before 1.0.6 allows local users to change ownership of arbitrary files via a symlink attack on ~/.Xauthority.
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Analysis
by VulDB Data Team • 11/30/2021
The vulnerability identified as CVE-2011-4105 affects LightDM versions prior to 1.0.6, representing a critical security flaw in the display manager component of Linux systems. This issue stems from improper handling of file permissions and symbolic link resolution during the Xauthority file management process. The vulnerability specifically targets the ~/.Xauthority file which serves as a crucial authentication mechanism for X Window System sessions, making it a prime target for privilege escalation attacks.
The technical flaw manifests through a symlink attack vector that exploits the insecure creation of the ~/.Xauthority file during LightDM session initialization. When LightDM starts a new session, it attempts to create the Xauthority file in the user's home directory without proper validation of existing symbolic links. An attacker with local access can create malicious symbolic links that point to sensitive system files, allowing them to manipulate file ownership and potentially gain elevated privileges. This weakness directly corresponds to CWE-377, which addresses insecure temporary file handling, and CWE-276, which covers improper file permissions. The vulnerability operates at the operating system level where file system permissions and access controls should prevent unauthorized modifications to critical system files.
The operational impact of this vulnerability extends beyond simple file ownership changes, as it can enable attackers to compromise the entire X Window System authentication framework. By manipulating the Xauthority file, an attacker could potentially impersonate other users or gain access to graphical sessions that should remain protected. This vulnerability particularly affects Linux distributions using LightDM as their default display manager, creating widespread exposure across numerous desktop environments and server configurations. The attack requires only local user access, making it particularly dangerous in multi-user environments where users might not be properly trusted. This aligns with ATT&CK technique T1068, which covers locally installed malicious software, and T1548.001, covering abuse of sudo privileges through insecure file handling.
Mitigation strategies for CVE-2011-4105 require immediate patching of LightDM to version 1.0.6 or later, which implements proper file permission checks and symbolic link validation. System administrators should also implement additional security measures including regular monitoring of the ~/.Xauthority file permissions, implementing proper file system access controls, and ensuring that display managers properly validate file creation operations. The vulnerability demonstrates the importance of secure file handling practices in authentication systems and highlights the need for proper privilege separation in display managers. Organizations should conduct comprehensive security audits of their display manager configurations and ensure that all system components properly validate file operations to prevent similar symlink attack vectors from compromising system integrity.