CVE-1999-1500 is a medium-severity vulnerability affecting Internet Anywhere POP3 Mail Server version 2.3.1. The vulnerability allows remote attackers to cause a denial of service (DoS) condition by sending malformed POP3 commands—specifically the LIST, TOP, or UIDL commands—with letter arguments instead of the expected numeric arguments. This input causes the mail server to crash, disrupting mail service availability. The vulnerability requires no authentication and can be exploited remotely over the network. The CVSS score of 5.0 reflects a moderate impact primarily on availability, with no impact on confidentiality or integrity. The attack vector is network-based with low complexity, and no user interaction is needed. Despite the age of this vulnerability (published in 1999), it remains relevant for legacy systems still running this outdated mail server software. No patches are available, and no known exploits have been reported in the wild, which may limit active exploitation but does not eliminate risk for unpatched, exposed servers.

For European organizations, the primary impact of this vulnerability is service disruption due to denial of service attacks on mail servers running Internet Anywhere POP3 Mail Server 2.3.1. Organizations relying on this legacy mail server software could experience mail outages, impacting business communications and potentially causing operational delays. While the vulnerability does not compromise data confidentiality or integrity, the loss of availability can affect customer service, internal communications, and compliance with service level agreements. The risk is higher for organizations with legacy infrastructure that have not migrated to modern mail solutions. Additionally, disruption of mail services could indirectly affect incident response and security monitoring capabilities if email is used for alerting or coordination.

Given the absence of official patches, European organizations should prioritize migrating away from Internet Anywhere POP3 Mail Server 2.3.1 to modern, supported mail server software that receives regular security updates. In the interim, organizations should implement network-level protections such as firewall rules to restrict access to the POP3 service only to trusted IP addresses and internal networks. Deploying intrusion detection/prevention systems (IDS/IPS) with signatures to detect malformed POP3 commands can help block exploit attempts. Monitoring mail server logs for unusual command usage patterns (e.g., LIST, TOP, UIDL commands with invalid arguments) can provide early warning of exploitation attempts. Additionally, organizations should consider isolating legacy mail servers in segmented network zones to limit potential impact. Regular security assessments should verify that no vulnerable instances remain exposed to the internet.