CVE-2001-0191 in gnuserv
Summary
by MITRE
gnuserv before 3.12, as shipped with XEmacs, does not properly check the specified length of an X Windows MIT-MAGIC-COOKIE cookie, which allows remote attackers to execute arbitrary commands via a buffer overflow, or brute force authentication by using a short cookie length.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 06/25/2021
The vulnerability identified as CVE-2001-0191 affects gnuserv versions prior to 3.12 that are distributed with XEmacs text editors. This represents a critical security flaw that exists within the X Windows authentication mechanism implementation. The vulnerability stems from insufficient validation of the MIT-MAGIC-COOKIE cookie length parameter, which is a fundamental component of X11 authentication protocols used for secure graphical sessions.
The technical flaw manifests as a buffer overflow condition within the gnuserv daemon when processing X Windows authentication requests. When a client attempts to authenticate with XEmacs through gnuserv, the system fails to properly validate the length of the MIT-MAGIC-COOKIE cookie provided by the client. This improper validation creates a scenario where an attacker can craft malicious authentication requests with oversized cookie data that exceeds the allocated buffer space. The buffer overflow vulnerability allows attackers to overwrite adjacent memory locations, potentially leading to arbitrary code execution within the context of the gnuserv process. This flaw directly maps to CWE-121, which describes heap-based buffer overflow conditions, and CWE-122, which covers stack-based buffer overflow scenarios.
The operational impact of this vulnerability extends beyond simple remote code execution capabilities. Attackers can leverage this weakness to gain unauthorized access to systems running vulnerable XEmacs installations, particularly those configured to accept remote gnuserv connections. The authentication brute force aspect of this vulnerability allows attackers to systematically test shorter cookie lengths to bypass authentication mechanisms, potentially gaining access to graphical environments without proper credentials. This creates a significant risk for systems where XEmacs is configured for network accessibility, as the vulnerability can be exploited without requiring prior authentication. The attack surface is particularly concerning in enterprise environments where X11 forwarding is commonly used for remote desktop access and development environments.
Mitigation strategies for this vulnerability require immediate patching of affected XEmacs installations to version 3.12 or later, which includes proper cookie length validation. System administrators should disable gnuserv functionality when not required, as this reduces the attack surface significantly. Network segmentation and firewall rules should be implemented to restrict access to XEmacs services to trusted networks only. The implementation of proper input validation and bounds checking for authentication cookies aligns with defensive coding practices recommended by the Software Engineering Institute's CERT Secure Coding Standards. Additionally, monitoring for unusual authentication patterns and implementing intrusion detection systems can help identify exploitation attempts. This vulnerability highlights the importance of proper authentication protocol implementation and demonstrates how seemingly minor validation flaws can result in severe security consequences, particularly when combined with the potential for privilege escalation through buffer overflow exploitation techniques.