CVE-2017-2625 in libXdmcp
Summary
by MITRE
It was discovered that libXdmcp before 1.1.2 including used weak entropy to generate session keys. On a multi-user system using xdmcp, a local attacker could potentially use information available from the process list to brute force the key, allowing them to hijack other users' sessions.
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Analysis
by VulDB Data Team • 04/27/2023
The vulnerability identified as CVE-2017-2625 affects the libXdmcp library version 1.1.2 and earlier, which is a critical component in X Window System implementations responsible for managing X Display Manager Control Protocol connections. This library serves as the foundation for remote desktop access and session management in Unix-like operating systems, making it a prime target for attackers seeking unauthorized access to graphical user interfaces. The flaw resides in the cryptographic implementation where the library utilizes weak entropy sources during the generation of session keys, fundamentally undermining the security of the authentication process. This weakness is particularly concerning because it directly impacts the confidentiality and integrity of user sessions in multi-user environments where XDMCP is actively employed.
The technical implementation of this vulnerability stems from the use of insufficiently random number generation algorithms within the session key creation process. When XDMCP establishes connections between display managers and clients, it requires secure session keys to maintain authentication integrity. However, the libXdmcp library prior to version 1.1.2 relied on predictable entropy sources that do not provide adequate randomness for cryptographic purposes. This weakness creates a scenario where an attacker with local access to the system can potentially reconstruct the session keys by analyzing process information, specifically from the process list where session key information may be exposed. The vulnerability essentially transforms what should be a cryptographically secure session establishment into a brute force attackable process, where the weak entropy reduces the key space to a manageable size for computational exploitation.
The operational impact of this vulnerability in real-world scenarios is particularly severe for multi-user systems that utilize XDMCP for remote desktop access and session management. Attackers with local privileges can leverage the predictable nature of the weak entropy to perform targeted brute force attacks against other users' sessions, potentially gaining unauthorized access to their graphical environments, executing arbitrary commands, and accessing sensitive data. This attack vector represents a significant escalation of privileges from a local user to potentially full system control, as graphical sessions often contain access to network resources, file systems, and application data that may not be available through command-line interfaces. The vulnerability also affects the principle of least privilege, as it allows local users to potentially impersonate other system users, undermining the fundamental security model of multi-user operating systems.
Mitigation strategies for this vulnerability require immediate patching of the libXdmcp library to version 1.1.2 or later, which implements proper cryptographic entropy sources for session key generation. System administrators should conduct comprehensive inventory assessments to identify all systems utilizing XDMCP and ensure that the patched versions are deployed across all affected environments. Additionally, organizations should consider implementing network segmentation to isolate XDMCP services from critical systems, as well as monitoring for suspicious process activity that might indicate brute force attempts against session keys. The vulnerability aligns with CWE-330, which addresses the use of insufficiently random values in cryptographic contexts, and represents a technique that could be categorized under ATT&CK tactic T1075, which involves establishing persistence through legitimate credentials. Organizations should also consider implementing additional security controls such as mandatory access controls, session monitoring, and periodic security audits to detect and prevent exploitation attempts. The remediation process should include verification that the patched library is properly installed and that no legacy versions remain accessible to prevent regression of the vulnerability.