CVE-1999-0626 is a vulnerability associated with the rpc.ruserd service, commonly known as 'rusers', which is a remote procedure call daemon used on Unix and Unix-like systems to provide information about users currently logged into a system. The vulnerability arises because certain versions of the rusers service expose valid user information to any entity on the network without requiring authentication or authorization. This means that an unauthenticated attacker on the same network can query the rusers service and retrieve a list of users currently logged into the system. While this does not directly compromise system integrity or availability, it leaks sensitive information about user presence and system usage patterns. The vulnerability is classified as low severity in the National Vulnerability Database (NVD) with a CVSS vector indicating network attack vector, low complexity, no authentication required, and no impact on confidentiality, integrity, or availability as per the CVSS score of 0. However, the exposure of user information can aid attackers in reconnaissance phases of an attack, potentially facilitating targeted social engineering, brute force attacks, or privilege escalation attempts. The vulnerability dates back to 1997 and affects the Sun Microsystems rpc.ruserd service, which was commonly used in older Unix environments. No patches are available, likely due to the age and obsolescence of the service. Modern systems typically do not run rpc.ruserd by default, but legacy systems or specialized environments may still have it enabled. The vulnerability does not require user interaction and can be exploited remotely over the network, making it accessible to any attacker with network access to the affected host.

For European organizations, the primary impact of CVE-1999-0626 is the unauthorized disclosure of user login information. While this does not directly compromise system security, it can provide attackers with valuable intelligence about active users, system usage times, and potentially user naming conventions. This information can be leveraged to craft more effective phishing campaigns, identify high-value targets, or attempt unauthorized access through password guessing or credential stuffing. In environments with strict privacy regulations such as GDPR, the exposure of user information—even just usernames—could be considered a data privacy concern, potentially leading to compliance issues. Additionally, organizations relying on legacy Unix systems or specialized industrial control systems that still run rpc.ruserd may be at risk. The vulnerability does not affect system integrity or availability, so direct operational impact is minimal. However, it increases the attack surface and can be a stepping stone for more serious attacks if combined with other vulnerabilities or weak security controls.

Given the age and nature of the vulnerability, the most effective mitigation is to disable the rpc.ruserd (rusers) service entirely if it is not required. This service is largely obsolete and provides limited functionality that can be replaced by more secure tools. For systems that must run rpc.ruserd, network-level controls should be implemented to restrict access to trusted hosts only, such as firewall rules limiting inbound traffic on the relevant RPC ports (typically port 111 for portmapper and ephemeral ports for rpc.ruserd). Network segmentation can also reduce exposure by isolating legacy systems from general user networks and the internet. Monitoring network traffic for unauthorized queries to rpc.ruserd can help detect reconnaissance attempts. If legacy systems cannot be updated or replaced, consider deploying host-based intrusion detection systems (HIDS) to alert on suspicious activity related to rpc.ruserd. Finally, organizations should conduct audits to identify any systems running this service and assess their necessity, removing or replacing them where possible.