CVE-1999-0626 in Rpc.ruserd
Summary
by MITRE
a version of rusers is running that exposes valid user information to any entity on the network.
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Analysis
by VulDB Data Team • 10/25/2025
The vulnerability described in CVE-1999-0626 pertains to the rusers daemon, a component of the remote user information service that was commonly found in Unix-based systems during the late 1990s. This daemon operates on TCP port 11 and provides information about currently logged-in users on a remote system to any client that connects to it. The flaw lies in the daemon's lack of authentication mechanisms, making it inherently insecure and accessible to any network entity that can reach the service. This vulnerability is classified under CWE-200, which deals with improper exposure of sensitive information, and represents a fundamental weakness in the system's security architecture where sensitive user data is exposed without proper access controls.
The technical implementation of this vulnerability stems from the rusers daemon's design philosophy that prioritized convenience over security. When a client connects to the rusers service, the daemon responds with a list of all currently logged-in users on the system, effectively providing a comprehensive user enumeration attack vector. This behavior violates the principle of least privilege and creates a significant information disclosure risk. The service operates without requiring authentication, authorization, or even encryption of the transmitted data, making it trivial for attackers to harvest user information from any system running the vulnerable version. The daemon essentially functions as an open door that reveals system user activity to any network participant, creating an information leak that could be exploited for further attacks.
The operational impact of this vulnerability extends far beyond simple information disclosure, as it provides attackers with crucial reconnaissance data that can be leveraged for more sophisticated attacks. An attacker who can access the rusers service gains knowledge of valid usernames on the system, which can then be used for password spraying attacks, brute force attempts, or social engineering campaigns. The vulnerability creates a pathway for attackers to identify potential targets within the system, understand user behavior patterns, and plan more effective infiltration strategies. From an attacker's perspective, this information is particularly valuable as it allows for targeted attacks against specific user accounts rather than random guessing. The vulnerability also enables network reconnaissance activities that can reveal the overall user base and activity patterns of the system, which can be used to map network topology and identify potential entry points. This type of attack aligns with techniques described in the MITRE ATT&CK framework under the reconnaissance phase, specifically targeting credential access and discovery capabilities.
Mitigation strategies for this vulnerability involve a combination of immediate operational changes and long-term architectural improvements. The most effective immediate solution is to disable or remove the rusers daemon from systems that do not require it, as the service provides no legitimate security benefit in modern network environments. System administrators should also implement network segmentation and access control lists to restrict access to port 11, preventing unauthorized access from external networks. Additionally, organizations should consider implementing firewall rules that block access to this service entirely, as the risk of exploitation far outweighs any potential benefit. The vulnerability demonstrates the importance of regularly auditing system services and removing unnecessary components that pose security risks. Organizations should also implement proper monitoring and logging of service access attempts to detect potential exploitation attempts. Security best practices dictate that services should be configured with the principle of least privilege, ensuring that only authorized users have access to sensitive information. The vulnerability serves as a reminder of the critical importance of secure configuration management and the need for regular security assessments to identify and remediate such exposure points.