CVE-1999-0420 in NetBSD
Summary
by MITRE
umapfs allows local users to gain root privileges by changing their uid through a malicious mount_umap program.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability described in CVE-1999-0420 resides within the umapfs filesystem implementation on unix-like operating systems, specifically targeting the privilege escalation mechanisms through improper user identification handling. This flaw exists in the mount_umap program which is responsible for managing user mapping within the umapfs filesystem. The vulnerability represents a classic privilege escalation issue where local users can manipulate their effective user identifiers to gain elevated privileges.
The technical flaw stems from insufficient validation within the umapfs subsystem that allows malicious users to modify their user identifier through crafted mount_umap operations. When a user executes the mount_umap program with specific parameters, the system fails to properly verify the legitimacy of the uid change request. This occurs because the implementation does not enforce proper access controls or authentication checks before accepting user identity modifications. The vulnerability exploits a fundamental security weakness in the privilege management system where the kernel does not adequately validate whether the requesting user has legitimate authorization to assume the target uid. This oversight creates an attack vector where any local user can potentially escalate their privileges to root level access.
The operational impact of this vulnerability is severe and far-reaching within affected systems. Local users who exploit this vulnerability can gain complete administrative control over the affected system, enabling them to modify system files, install malicious software, create new user accounts, and access sensitive data. The vulnerability is particularly dangerous because it requires no special privileges to initiate the attack, making it accessible to any user with login access to the system. Additionally, the attack does not require network connectivity or external tools beyond the standard mount_umap program, making it easily exploitable in various environments. The privilege escalation occurs at the kernel level, bypassing standard user permission checks and providing attackers with unrestricted system access.
Mitigation strategies for this vulnerability must address both the immediate security flaw and the underlying design issues within the umapfs implementation. System administrators should immediately disable or remove the umapfs filesystem from production systems where it is not absolutely required, as this represents the most effective immediate remediation. The system should also be updated with patches that implement proper uid validation and access control checks within the mount_umap program. Security configurations should enforce strict auditing of mount_umap operations and user identity changes, implementing logging mechanisms that can detect suspicious privilege escalation attempts. Organizations should also consider implementing mandatory access controls and privilege separation mechanisms to limit the potential impact of such vulnerabilities. The vulnerability aligns with CWE-276 which describes improper privilege management and represents a classic example of privilege escalation through improper access control, while also mapping to ATT&CK technique T1068 which covers privilege escalation through local exploitation of system vulnerabilities. Regular security audits should verify that no unauthorized users have access to the vulnerable mount_umap program and that system configurations properly enforce least privilege principles.