CVE-1999-1500 in Internet Anywhere Mail Server
Summary
by MITRE
Internet Anywhere POP3 Mail Server 2.3.1 allows remote attackers to cause a denial of service (crash) via (1) LIST, (2) TOP, or (3) UIDL commands using letters as arguments.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability described in CVE-1999-1500 affects the Internet Anywhere POP3 Mail Server version 2.3.1, representing a classic denial of service flaw that exploits improper input validation in core mail server protocols. This issue specifically targets the POP3 protocol implementation where the server fails to properly handle malformed commands containing alphabetic characters as arguments. The vulnerability exists within the server's command processing logic where it does not adequately validate the format of arguments passed to fundamental POP3 commands including LIST, TOP, and UIDL. These commands are essential for email clients to retrieve message listings, retrieve specific message content, and access unique message identifiers respectively, making their reliable operation critical for email service availability.
The technical flaw manifests when remote attackers send specially crafted POP3 commands with letter-based arguments to the vulnerable server. The server processes these commands without proper sanitization or validation, leading to memory corruption or stack overflow conditions that ultimately result in the server crashing and becoming unavailable to legitimate users. This type of vulnerability falls under the broader category of improper input validation as classified by CWE-20, where the system fails to properly validate or sanitize input received from untrusted sources. The vulnerability specifically demonstrates weaknesses in protocol parsing and error handling mechanisms, where the server's failure to gracefully handle unexpected input formats leads to complete service disruption rather than merely returning error responses.
The operational impact of this vulnerability is significant as it allows remote attackers to disrupt email services without requiring authentication or specialized privileges. The denial of service condition affects the availability of email services for legitimate users, potentially causing business disruption and communication failures. Email servers that implement the affected POP3 protocol are particularly vulnerable since these commands are commonly used by email clients during normal operation, meaning the attack can be executed with minimal detection. The vulnerability's exploitation requires only basic network connectivity and knowledge of the POP3 protocol, making it accessible to attackers with limited technical expertise. This characteristic aligns with ATT&CK technique T1499.004 for network denial of service attacks, where adversaries leverage protocol-level weaknesses to compromise service availability.
Mitigation strategies for this vulnerability include implementing proper input validation and sanitization for all POP3 command arguments, ensuring that the server rejects malformed commands before attempting to process them. System administrators should consider applying patches or updates from the vendor to address the specific implementation flaw in Internet Anywhere POP3 Mail Server 2.3.1. Network-level protections such as firewall rules that limit POP3 access to trusted networks and implementing rate limiting for command processing can help reduce the impact of potential attacks. Additionally, monitoring for unusual command patterns and implementing robust error handling mechanisms that prevent crashes from occurring during malformed input processing would provide defense in depth against similar vulnerabilities. Organizations should also consider migrating to more modern mail server implementations that have better input validation and robust error handling mechanisms, as the affected version represents an older implementation with known security weaknesses that are unlikely to receive further security updates.