CVE-2009-1756 in SLiM Simple Login Manager
Summary
by MITRE
SLiM Simple Login Manager 1.3.0 places the X authority magic cookie (mcookie) on the command line when invoking xauth from (1) app.cpp and (2) switchuser.cpp, which allows local users to access the X session by listing the process and its arguments.
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Analysis
by VulDB Data Team • 09/05/2019
The vulnerability identified as CVE-2009-1756 affects SLiM Simple Login Manager version 1.3.0, a widely used display manager for Unix-like systems. This flaw represents a critical security oversight in how the login manager handles X11 authentication tokens, specifically the X authority magic cookie known as mcookie. The vulnerability stems from the insecure practice of passing sensitive authentication data directly through command-line arguments when executing xauth commands. This design flaw creates a significant attack surface that undermines the fundamental security model of X Window System authentication mechanisms.
The technical implementation of this vulnerability occurs in two primary source files within the SLiM codebase: app.cpp and switchuser.cpp. When the login manager needs to manage X session authentication, it invokes xauth commands with the mcookie value passed as a command-line argument rather than through secure stdin or file-based mechanisms. This approach violates established security principles and creates an exploitable condition where any local user can enumerate running processes and extract the complete command line arguments. The X authority magic cookie serves as the primary authentication token that grants access to the X server, making it equivalent to a password or cryptographic key in the authentication chain. The exposure of this token through process listings fundamentally compromises the security of the graphical session.
From an operational perspective, this vulnerability enables local privilege escalation and session hijacking attacks with minimal technical expertise required. Any user with access to the system can simply execute standard process enumeration commands such as ps aux or similar utilities to discover the command-line arguments of running processes. The mcookie value becomes immediately accessible in the process arguments, allowing attackers to forge X11 authentication tokens and gain unauthorized access to active graphical sessions. This weakness is particularly dangerous in multi-user environments where users may be running graphical applications that require X11 access, as it enables attackers to monitor, control, or steal data from other users' sessions. The attack vector is particularly concerning because it requires no special privileges beyond basic local user access and can be executed using standard system administration tools.
The vulnerability aligns with CWE-255 Improper Handling of Credentials, specifically addressing the improper handling of authentication tokens through command-line arguments. This weakness creates a direct pathway for privilege escalation attacks and violates fundamental security principles outlined in the MITRE ATT&CK framework under T1068, which covers the abuse of elevated privileges and session management. The attack surface extends beyond simple session access to include potential data exfiltration, screen capture, and manipulation of graphical applications running under other user accounts. Organizations using SLiM 1.3.0 are particularly vulnerable as this flaw affects the core authentication mechanism of the display manager, making it a critical security concern that requires immediate remediation through software updates or configuration changes.
Mitigation strategies for this vulnerability should focus on immediate software updates to versions that properly handle X authority tokens through secure channels rather than command-line arguments. System administrators should implement process monitoring to detect and alert on suspicious command-line patterns involving xauth or similar authentication utilities. The recommended remediation approach involves patching the SLiM software to eliminate the direct command-line exposure of mcookie values, typically by using file-based authentication mechanisms or secure stdin handling. Organizations should also consider implementing additional security controls such as process isolation, privilege separation, and monitoring for unauthorized access to X11 sessions. The vulnerability demonstrates the importance of secure coding practices and the critical need for proper credential handling in authentication systems, particularly those that interface with privileged system components like X Window System access controls.