CVE-1999-0731 in Openlinux
Summary
by MITRE
The KDE klock program allows local users to unlock a session using malformed input.
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Analysis
by VulDB Data Team • 04/19/2026
The CVE-1999-0731 vulnerability resides within the KDE klock program, a screen locker component of the K Desktop Environment that was widely deployed in Unix and Linux systems during the late 1990s. This security flaw represents a classic authentication bypass vulnerability that exploited improper input validation mechanisms within the session unlocking process. The vulnerability specifically manifests when the klock program processes malformed input data during session unlock attempts, potentially allowing unauthorized local users to circumvent the normal authentication procedures required to access locked desktop sessions. This issue directly impacts the fundamental security model of desktop environments where session locking serves as a primary defense mechanism against unauthorized access to user workspaces.
The technical implementation of this vulnerability stems from insufficient validation of user input within the klock program's authentication routines. When users attempt to unlock their sessions, the program processes input through a parsing mechanism that fails to properly sanitize or validate the format of unlock credentials. This weakness creates a path where malformed input can be interpreted in unexpected ways, potentially triggering code paths that bypass normal authentication checks. The vulnerability operates at the application level rather than at the system or kernel level, making it a local privilege escalation issue that requires physical access or existing user privileges to exploit. According to CWE classification, this vulnerability maps to CWE-20: Improper Input Validation, which encompasses issues where applications fail to properly validate input data before processing it. The flaw demonstrates characteristics consistent with buffer overflow conditions or parsing errors that could lead to arbitrary code execution or authentication bypass.
From an operational impact perspective, this vulnerability poses significant risks to desktop security environments where multiple users share systems or where physical security controls are inadequate. Local users who can access the system console or have basic user privileges can exploit this weakness to gain unauthorized access to locked sessions, potentially accessing sensitive data, applications, or system resources that were intended to be protected. The vulnerability affects systems running KDE desktop environments from version 1.0 through 2.1, which were prevalent in enterprise and academic computing environments during the late 1990s and early 2000s. The attack vector requires local system access and typically does not allow remote exploitation, but the implications are severe for environments where physical security is not adequately maintained. This vulnerability aligns with ATT&CK technique T1547.001: Registry Run Keys / Startup Folder, as it exploits legitimate system components to gain unauthorized access to protected sessions, though the specific technique is more accurately classified under privilege escalation methods.
The mitigation strategies for CVE-1999-0731 involve immediate patching of affected KDE installations through updated klock binaries or complete KDE environment upgrades. System administrators should ensure that all KDE desktop environments are updated to versions that include proper input validation for session unlock mechanisms. Additional protective measures include implementing proper system access controls, monitoring for unusual unlock patterns, and ensuring that physical security measures are adequate to prevent unauthorized local access. The vulnerability highlights the importance of input validation in authentication systems and demonstrates how seemingly minor flaws in desktop security components can create significant risks. Organizations should also consider implementing additional security layers such as encrypted storage, secure session management, and regular security assessments of desktop environments to prevent similar issues from arising in modern systems. Given the age of this vulnerability, contemporary systems are unlikely to be affected by the specific implementation details, but the underlying principles of input validation and authentication security remain relevant to modern security practices and continue to inform the development of secure authentication mechanisms across various computing platforms.