CVE-2000-0763 in xlock
Summary
by MITRE
xlockmore and xlockf do not properly cleanse user-injected format strings, which allows local users to gain root privileges via the -d option.
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Analysis
by VulDB Data Team • 01/21/2025
The vulnerability identified as CVE-2000-0763 affects xlockmore and xlockf programs, which are screen locking utilities commonly found in unix-like operating systems. These applications are designed to lock the screen and require authentication to unlock, serving as basic security measures to protect systems when users are away from their workstations. The flaw resides in how these programs handle command-line arguments, specifically when processing the -d option which is intended to specify the display device. When users provide maliciously crafted format strings through this argument, the applications fail to properly sanitize the input before using it in printf-style functions, creating a classic format string vulnerability that can be exploited by local attackers.
This vulnerability operates through a format string exploitation technique where an attacker can inject specially crafted input that contains format specifiers such as %s, %d, or %x. When the vulnerable program processes these inputs without proper validation or cleansing, it interprets the format specifiers as instructions to read from memory locations or write data to specific addresses. The attack can be leveraged to read sensitive information from memory, overwrite critical program variables, or even execute arbitrary code with elevated privileges. Since these screen locking utilities typically run with elevated permissions to properly lock the display and manage system resources, a successful exploitation can result in privilege escalation from the local user account to the root user level.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it represents a fundamental flaw in input validation within security-critical system components. Attackers can exploit this weakness to gain complete control over the affected system, potentially leading to data theft, system compromise, or use as a foothold for further attacks within a network. The vulnerability is particularly concerning because screen locking utilities are often run with elevated privileges to perform their core functions, making them attractive targets for privilege escalation attacks. This type of vulnerability aligns with CWE-134, which describes the weakness of using user-controlled format strings in functions like printf, and can be mapped to ATT&CK technique T1068, which covers privilege escalation through local exploitation of system vulnerabilities.
Mitigation strategies for this vulnerability involve implementing proper input validation and sanitization for all user-provided arguments, particularly those used in printf-style functions. System administrators should ensure that affected versions of xlockmore and xlockf are updated to patched versions that properly handle format strings, or alternatively disable the problematic -d option if it is not essential for system operation. Additionally, implementing proper privilege separation and using more secure alternatives for screen locking can reduce the attack surface. The vulnerability demonstrates the importance of input validation in security-critical applications and highlights how seemingly benign functionality can create significant security risks when not properly secured against format string attacks. Organizations should conduct regular vulnerability assessments to identify similar weaknesses in other system utilities and ensure that all applications properly validate and sanitize user input before processing it in any context that could lead to privilege escalation or information disclosure.