CVE-1999-0656 in Linux
Summary
by MITRE
The ugidd RPC interface, by design, allows remote attackers to enumerate valid usernames by specifying arbitrary UIDs that ugidd maps to local user and group names.
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Analysis
by VulDB Data Team • 02/12/2025
The CVE-1999-0656 vulnerability resides within the ugidd RPC interface, a component that serves as a user and group identity mapping service. This interface operates by translating user identifiers from remote systems into local user and group names on the target system. The fundamental flaw lies in the interface's design approach that permits arbitrary UID specification without proper validation or access control mechanisms. Attackers can exploit this weakness by sending crafted RPC requests with varying UIDs to probe the system and discover valid local usernames and group memberships.
The technical implementation of this vulnerability stems from inadequate input validation within the ugidd service. When remote clients connect to the RPC interface, they can provide any UID value they choose, and the service will attempt to map these identifiers to corresponding local user accounts. This design decision creates a direct information disclosure channel where attackers can systematically test different UID values to enumerate legitimate local accounts. The service essentially acts as a lookup table that reveals which UIDs correspond to valid local users, providing attackers with valuable reconnaissance information for subsequent attacks.
The operational impact of this vulnerability extends beyond simple information disclosure, as it significantly weakens the overall security posture of systems running the affected service. Security researchers have categorized this type of vulnerability under CWE-200, which addresses "Information Disclosure," and it aligns with ATT&CK technique T1087.001 for account discovery. The enumeration capability allows threat actors to build comprehensive user dictionaries that can be used for password spraying attacks, brute force attempts, or social engineering operations. This vulnerability particularly affects systems where the ugidd service is exposed to untrusted networks or where proper network segmentation has not been implemented.
Mitigation strategies for CVE-1999-0656 should focus on both immediate operational fixes and architectural improvements. Organizations should disable or restrict access to the ugidd RPC interface to trusted networks only, implementing proper firewall rules and network segmentation. The service should be configured to validate input parameters and reject arbitrary UID specifications that fall outside expected ranges or that do not correspond to legitimate local accounts. Additionally, system administrators should consider implementing access control lists that limit which remote systems can query the interface and should monitor RPC traffic for suspicious enumeration patterns. From a compliance perspective, this vulnerability would violate security standards such as those outlined in NIST SP 800-53 and ISO 27001, which require proper access controls and information protection mechanisms. The most effective long-term solution involves either replacing the vulnerable service with more secure alternatives or implementing proper authentication and authorization controls that prevent unauthorized enumeration attempts.