CVE-1999-1504 is a vulnerability found in Stalker Internet Mail Server version 1.6, which allows a remote attacker to cause a denial of service (DoS) condition by sending an excessively long HELO command during the SMTP handshake process. The HELO command is part of the SMTP protocol used by mail servers to identify themselves when initiating a mail session. In this case, the server does not properly handle or validate the length of the HELO command, leading to a crash or service disruption. This vulnerability is exploitable remotely without authentication, as it only requires the attacker to establish a connection to the mail server and send the malformed HELO command. The CVSS score of 5.0 (medium severity) reflects that the impact is limited to availability, with no direct impact on confidentiality or integrity. There is no patch available for this vulnerability, and no known exploits have been reported in the wild. Given the age of the vulnerability (published in 1998) and the specific affected product, this issue primarily affects legacy systems still running Stalker Internet Mail Server 1.6. The lack of authentication requirement and ease of exploitation through a simple malformed command make this a straightforward DoS attack vector, potentially disrupting email services hosted on vulnerable servers.

For European organizations, the primary impact of this vulnerability is the potential disruption of email services, which can affect business communications and operational continuity. Organizations relying on legacy mail servers like Stalker Internet Mail Server 1.6 may experience service outages if targeted by attackers exploiting this vulnerability. Although the vulnerability does not compromise data confidentiality or integrity, the denial of service can lead to loss of productivity, delayed communications, and potential reputational damage if email services are critical to customer or partner interactions. Additionally, prolonged downtime could affect compliance with regulations requiring timely communication or incident reporting. However, given the age and obscurity of the affected software, the overall risk to most European organizations is low unless they operate legacy systems without modern mail server replacements or mitigations.

Since no official patch is available for this vulnerability, European organizations should consider the following specific mitigation steps: 1) Immediately identify and inventory any instances of Stalker Internet Mail Server 1.6 in their environment. 2) Replace or upgrade the mail server software to a modern, supported mail server solution that properly validates SMTP commands and is actively maintained. 3) If replacement is not immediately feasible, implement network-level protections such as firewall rules or intrusion prevention systems (IPS) to detect and block malformed SMTP commands, particularly excessively long HELO commands. 4) Employ rate limiting and connection throttling on SMTP ports to reduce the risk of DoS attacks. 5) Monitor mail server logs and network traffic for unusual SMTP command patterns indicative of exploitation attempts. 6) Segment legacy mail servers from critical network segments to limit the impact of potential service disruptions. These targeted mitigations go beyond generic advice by focusing on compensating controls and proactive detection tailored to this specific vulnerability and its exploitation vector.