CVE-1999-1266 is a vulnerability in the rsh daemon (rshd), a service that allows remote shell access to Unix-like systems. The issue arises because rshd generates different error messages depending on whether a username provided during a connection attempt is valid or invalid. Specifically, when a valid username is supplied, the daemon responds with one type of error message, and when an invalid username is provided, it responds with a different message. This discrepancy allows remote attackers to enumerate valid usernames on the target system without authentication. User enumeration is a reconnaissance technique that can facilitate further attacks, such as brute force password guessing or social engineering. The vulnerability has a CVSS v2 base score of 5.0, indicating medium severity, with an attack vector of network (remote), low attack complexity, no authentication required, and partial confidentiality impact (disclosure of valid usernames). There is no impact on integrity or availability. No patches are available, and there are no known exploits in the wild. The affected product is identified as 'metamail' by Metamail Corporation, but the vulnerability specifically concerns the rsh daemon component. Given the age of the vulnerability (published in 1997) and the obsolescence of rsh in favor of more secure protocols like SSH, this issue is primarily relevant in legacy environments where rshd is still in use. However, in such environments, the ability to enumerate valid users remotely can aid attackers in mounting further attacks.

For European organizations, the primary impact of this vulnerability is the exposure of valid usernames on systems running the rsh daemon. This information leakage can facilitate targeted brute force attacks or credential stuffing campaigns, potentially leading to unauthorized access if weak or reused passwords are present. Although the vulnerability does not directly allow system compromise or denial of service, it lowers the barrier for attackers to identify valid accounts, increasing the risk of subsequent attacks. Organizations in sectors with legacy Unix systems or those that have not migrated away from rsh may be particularly at risk. The confidentiality of user account information is compromised, which may have compliance implications under regulations such as GDPR if user identity information is exposed. However, the lack of known exploits and the medium severity rating suggest that the immediate risk is moderate. The vulnerability is less likely to impact modern environments that have replaced rsh with SSH or other secure protocols.

Given that no patches are available for this vulnerability, European organizations should focus on mitigating risk through configuration and operational controls. First, disable the rsh daemon entirely and replace it with secure alternatives such as SSH, which provides encrypted communication and does not leak user enumeration information. If disabling rsh is not immediately feasible, restrict access to the rsh service using network-level controls such as firewalls or TCP wrappers to limit connections to trusted hosts only. Additionally, implement strong password policies and account lockout mechanisms to reduce the risk of brute force attacks leveraging enumerated usernames. Monitoring and logging of authentication attempts should be enhanced to detect suspicious activity related to user enumeration or repeated login failures. For legacy systems that must retain rsh, consider deploying intrusion detection systems capable of identifying reconnaissance attempts. Finally, conduct regular audits to identify any systems still running rshd and prioritize their upgrade or decommissioning to reduce exposure.