CVE-2011-3349 in LightDM
Summary
by MITRE
lightdm before 0.9.6 writes in .dmrc and Xauthority files using root permissions while the files are in user controlled folders. A local user can overwrite root-owned files via a symlink, which can allow possible privilege escalation.
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Analysis
by VulDB Data Team • 02/23/2024
The vulnerability identified as CVE-2011-3349 affects lightdm versions prior to 0.9.6 and represents a critical privilege escalation flaw in the display manager's file handling mechanisms. This issue stems from improper permission handling during the creation and modification of configuration files within user-accessible directories. The vulnerability specifically manifests when lightdm writes to .dmrc and Xauthority files, which are typically stored in user-controlled locations such as the home directory. The flaw occurs because the display manager executes file operations with root privileges while operating in user contexts, creating a dangerous mismatch between file ownership expectations and actual system permissions.
The technical exploitation of this vulnerability relies on a classic symlink attack pattern where a local user can manipulate the file system to create symbolic links that point to root-owned files in critical system locations. When lightdm attempts to write to the .dmrc or Xauthority files, it follows these symbolic links and inadvertently overwrites root-owned files with data controlled by the attacker. This behavior violates fundamental security principles of least privilege and file system isolation, as the system grants elevated permissions to operations that should remain constrained to user-level access. The vulnerability falls under the CWE-276 category of insecure file permissions and is closely related to CWE-284 which addresses improper access control mechanisms.
From an operational impact perspective, this vulnerability enables local users to potentially escalate their privileges to root level access, which represents a severe security compromise. Attackers can leverage this flaw to overwrite critical system files, modify authentication mechanisms, or establish persistent backdoors within the system. The attack requires local system access and basic user privileges, making it particularly dangerous in multi-user environments where users might not be trusted. The vulnerability affects systems using lightdm as their display manager, which was commonly found in Ubuntu and other debian-based distributions during the affected time period. The impact extends beyond simple privilege escalation as it can facilitate further attacks including lateral movement within networked environments and complete system compromise.
Mitigation strategies for this vulnerability involve immediate patching of lightdm to version 0.9.6 or later, which implements proper file permission handling and prevents the use of root privileges during file creation operations in user-controlled directories. System administrators should also implement additional controls such as monitoring for unauthorized file modifications in critical system locations and ensuring proper file permissions are maintained on .dmrc and Xauthority files. The fix addresses the underlying issue by ensuring that file operations occur with appropriate user-level permissions rather than elevated privileges, aligning with the principle of least privilege. Additionally, organizations should conduct regular security audits to identify similar permission-related vulnerabilities in other system components and implement automated monitoring solutions to detect potential exploitation attempts. This vulnerability demonstrates the importance of proper privilege separation and file system security controls in display managers and authentication systems.