CVE-1999-0418 in Host
Summary
by MITRE
Denial of service in SMTP applications such as Sendmail, when a remote attacker (e.g. spammer) uses many "RCPT TO" commands in the same connection.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability described in CVE-1999-0418 represents a classic denial of service attack targeting Simple Mail Transfer Protocol implementations, particularly affecting prominent mail servers like sendmail. This weakness exploits the fundamental communication patterns within SMTP protocols where clients establish connections to mail servers to transmit email messages. The flaw specifically manifests when a remote attacker leverages the RCPT TO command sequence to overwhelm server resources, creating a scenario where legitimate email services become unavailable to authorized users.
The technical implementation of this vulnerability stems from insufficient input validation and resource management within SMTP server applications. When an attacker sends multiple RCPT TO commands during a single SMTP session without properly closing the connection, the server maintains state information for each recipient address. This behavior creates a resource exhaustion condition where server memory and processing capacity become consumed by tracking numerous recipient addresses, ultimately preventing the server from accepting new connections or processing legitimate email transactions. The vulnerability operates at the application layer of network communication, making it particularly effective against mail servers that do not implement proper rate limiting or connection management controls.
From an operational perspective, this vulnerability poses significant risks to email infrastructure reliability and service availability. Organizations relying on affected SMTP implementations face potential disruption of critical communication channels, with attackers able to render mail servers completely unresponsive through relatively simple command sequences. The impact extends beyond immediate service disruption to include potential business continuity issues, as email remains a fundamental communication mechanism for most organizations. The vulnerability's exploitation requires minimal technical skill and can be automated, making it particularly dangerous for widespread deployment by malicious actors including spammers and cybercriminals seeking to disrupt email services.
Security practitioners should implement several mitigation strategies to address this vulnerability, including configuring rate limiting on recipient commands, implementing connection timeouts, and establishing proper resource allocation limits for SMTP sessions. The vulnerability aligns with CWE-400, which categorizes excessive resource consumption as a fundamental weakness in software design, and corresponds to ATT&CK technique T1499.004, which describes network denial of service attacks. Organizations should also deploy monitoring solutions to detect unusual patterns of RCPT TO command sequences and implement proper access controls to limit the number of simultaneous connections from individual sources. Network segmentation and firewall rules can further limit the impact by restricting access to mail server resources and implementing connection throttling mechanisms that prevent single clients from consuming excessive server resources.