CVE-1999-0901 in Linux
Summary
by MITRE
ypserv allows a local user to modify the GECOS and login shells of other users.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability described in CVE-1999-0901 resides within the ypserv implementation, which is part of the NIS (Network Information Service) system used for centralized network administration. This flaw represents a significant privilege escalation vector that allows local users to manipulate user account information across the network. The GECOS field contains user identification information including full names and contact details, while login shells determine the command interpreter used when users log in to the system. When ypserv fails to properly validate user permissions or enforce access controls, it creates an opportunity for malicious local users to modify these critical account attributes for other users.
This vulnerability operates at the intersection of system security and network service management, specifically targeting the authentication and authorization mechanisms that should protect user account integrity. The technical flaw manifests as insufficient input validation and access control checks within the ypserv daemon, which processes requests to update NIS user maps. Local attackers can exploit this weakness to modify user attributes without proper authorization, potentially gaining elevated privileges or disrupting normal user operations. The vulnerability's classification aligns with CWE-264, which covers permissions, privileges, and access control issues, particularly focusing on inadequate access control mechanisms in network services.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it can enable attackers to manipulate user account configurations in ways that may facilitate further attacks. By modifying login shells, attackers could potentially redirect users to malicious command interpreters or disable legitimate access methods. Similarly, altering GECOS fields can obscure user identities and make forensic analysis more difficult. The vulnerability affects systems where NIS is actively used for user account management, which was common in enterprise environments during the late 1990s and early 2000s. This type of attack represents a classic case of insufficient privilege separation, where local users with minimal access can undermine the integrity of centralized user management systems.
Mitigation strategies for CVE-1999-0901 should focus on implementing proper access controls and system hardening measures for NIS services. System administrators should ensure that ypserv runs with minimal necessary privileges and that proper authentication mechanisms are enforced for all user modification operations. The implementation of mandatory access controls and regular security auditing of NIS configurations can help detect unauthorized modifications. Additionally, organizations should consider migrating away from NIS in favor of more modern authentication systems like LDAP or Kerberos, which provide better access control mechanisms and are less prone to similar vulnerabilities. The ATT&CK framework categorizes this vulnerability under privilege escalation techniques, specifically targeting service execution and credential access phases, where attackers seek to modify system configurations to gain persistent access or manipulate user accounts for malicious purposes.