CVE-2000-0620 in X11r6
Summary
by MITRE
libX11 X library allows remote attackers to cause a denial of service via a resource mask of 0, which causes libX11 to go into an infinite loop.
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Analysis
by VulDB Data Team • 06/08/2017
The vulnerability identified as CVE-2000-0620 resides within the libX11 X library, a fundamental component of the X Window System that manages graphical user interfaces on Unix-like operating systems. This flaw represents a classic denial of service vulnerability that can be exploited remotely by malicious actors to disrupt system operations. The libX11 library serves as a crucial interface between client applications and the X server, making it a prime target for attackers seeking to compromise system availability. The vulnerability specifically manifests when the library encounters a resource mask value of zero, which triggers an unexpected behavior in the library's processing logic.
The technical root cause of this vulnerability lies in the improper handling of resource mask values within the libX11 library implementation. When a remote attacker sends a specially crafted X protocol message containing a resource mask set to zero, the library's internal processing routine enters an infinite loop condition. This occurs because the library fails to validate or properly handle the edge case where the resource mask equals zero, causing the program flow to repeatedly execute the same code path without proper termination conditions. The flaw demonstrates poor input validation and error handling practices that are commonly associated with buffer overflow and resource management vulnerabilities.
From an operational perspective, this vulnerability presents a significant risk to systems relying on the X Window System for graphical interface management. Remote attackers can exploit this weakness to cause sustained denial of service conditions, effectively rendering graphical applications and user interfaces unavailable. The infinite loop consumes system resources and processing power, potentially leading to system instability or complete service disruption. This vulnerability affects a wide range of Unix-like systems including various Linux distributions, BSD variants, and other operating systems that utilize the X Window System. The remote exploit capability means that attackers do not need physical access or local privileges to trigger the vulnerability, making it particularly dangerous in networked environments.
The impact of this vulnerability aligns with CWE-835, which addresses the issue of infinite loops in software implementations. This classification reflects the fundamental flaw in the library's logic where the absence of proper termination conditions creates a path that cannot be exited naturally. From an attack framework perspective, this vulnerability would be categorized under the denial of service category in the MITRE ATT&CK framework, specifically targeting system availability through resource exhaustion. The exploitability of this vulnerability is relatively straightforward, requiring only basic knowledge of the X protocol and the ability to send crafted network packets to the target system's X server.
Mitigation strategies for this vulnerability involve immediate patching of the libX11 library to implement proper input validation and termination conditions for resource mask values. System administrators should prioritize updating their X Window System components to versions that contain fixes for this specific issue. Network-level defenses can include firewall rules that restrict access to X server ports, particularly in environments where remote access is not required. Additionally, implementing monitoring solutions to detect unusual resource consumption patterns can help identify exploitation attempts. The vulnerability serves as a reminder of the importance of robust input validation and proper error handling in system libraries, particularly those handling network communications and user input. Organizations should conduct regular security assessments of their X Window System implementations and maintain up-to-date security patches to prevent similar vulnerabilities from compromising system availability.