CVE-2003-0549 in GDM
Summary
by MITRE
The X Display Manager Control Protocol (XDMCP) support for GDM before 2.4.1.6 allows attackers to cause a denial of service (daemon crash) via a short authorization key name.
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Analysis
by VulDB Data Team • 06/29/2021
The vulnerability described in CVE-2003-0549 represents a critical denial of service flaw within the X Display Manager Control Protocol implementation of the GNOME Display Manager. This issue specifically affects GDM versions prior to 2.4.1.6 and stems from inadequate input validation mechanisms within the XDMCP protocol handling code. The flaw manifests when an attacker crafts a malformed authorization key name that is shorter than expected, causing the daemon to crash and terminate its operations. This type of vulnerability falls under the category of improper input validation as classified by CWE-20, which encompasses weaknesses that arise from insufficient validation of input data. The XDMCP protocol serves as a critical component in graphical login systems, enabling remote management and control of display managers, making this vulnerability particularly concerning for systems that rely on remote desktop access.
The technical exploitation of this vulnerability occurs through the manipulation of authorization key names during the XDMCP communication process. When GDM receives an authorization request containing an authorization key name that is shorter than the expected minimum length, the daemon fails to properly handle this condition, leading to a memory access violation or buffer underflow. This improper handling causes the display manager daemon to terminate unexpectedly, effectively denying legitimate users access to the graphical login interface. The flaw demonstrates a classic buffer handling weakness where the system does not adequately check the length of incoming authorization data before processing it. According to ATT&CK framework, this vulnerability maps to T1499.004 which covers "Network Denial of Service" techniques, as the attack specifically targets the availability of network services through daemon termination.
The operational impact of this vulnerability extends beyond simple service disruption, as it can be leveraged to create persistent denial of service conditions that affect entire desktop environments. Systems using GDM for graphical login management become vulnerable to attacks that can be executed remotely, potentially allowing unauthorized users to repeatedly crash the display manager and prevent legitimate access to the system. This type of attack can be particularly problematic in multi-user environments where the display manager serves as the primary interface for system access. Organizations relying on GNOME desktop environments for their computing infrastructure face significant risk from this vulnerability, as it can be exploited without requiring elevated privileges or complex attack vectors. The vulnerability also highlights the importance of proper error handling in daemon processes, as the system should gracefully handle malformed input rather than crashing completely.
Mitigation strategies for this vulnerability require immediate patching of affected GDM installations to versions 2.4.1.6 or later, where the input validation has been corrected. System administrators should also implement network-level controls to restrict access to XDMCP ports, typically port 177, thereby reducing the attack surface available to potential attackers. Additional defensive measures include implementing monitoring solutions that can detect unusual daemon termination patterns and establishing automated restart procedures for display manager services. The vulnerability underscores the importance of robust input validation and error handling in critical system components, as recommended by security best practices and standards such as those outlined in the OWASP Top Ten. Organizations should also consider implementing intrusion detection systems that can identify and alert on suspicious XDMCP traffic patterns that may indicate exploitation attempts. Regular security assessments and vulnerability scanning should be conducted to identify similar input validation weaknesses in other system components that may present similar risks.