CVE-1999-0419 in Exchange
Summary
by MITRE
When the Microsoft SMTP service attempts to send a message to a server and receives a 4xx error code, it quickly and repeatedly attempts to redeliver the message, causing a denial of service.
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Analysis
by VulDB Data Team • 04/18/2026
The vulnerability described in CVE-1999-0419 represents a classic denial of service flaw within Microsoft's SMTP service implementation that emerged during the late 1990s era of email infrastructure development. This weakness specifically targets the message delivery retry mechanism of the SMTP service, where the system fails to properly handle transient network errors that result in 4xx status codes during message transmission. The flaw operates at the protocol level where the service interprets these error responses as temporary failures rather than permanent delivery issues, leading to an infinite loop of retry attempts.
The technical implementation of this vulnerability stems from inadequate error handling within the SMTP service's delivery logic. When the service encounters a 4xx error code from a remote mail server, it does not implement proper exponential backoff or maximum retry limits. Instead, it immediately attempts redelivery without sufficient delay or circuit breaking mechanisms, creating a scenario where the service becomes trapped in rapid successive retry cycles. This behavior directly violates the principles of robust network service design as outlined in various cybersecurity frameworks and represents a failure to implement proper error recovery patterns that are fundamental to resilient system architecture.
The operational impact of this vulnerability extends beyond simple service disruption to create significant resource exhaustion conditions within the affected system. The rapid succession of retry attempts consumes substantial CPU cycles and network bandwidth, potentially leading to complete system unresponsiveness or performance degradation that affects legitimate email traffic. Network administrators would observe unusual traffic patterns and system resource consumption spikes, with the affected SMTP service becoming unable to process new incoming messages while simultaneously attempting to redeliver failed messages. This creates a cascading effect where the service becomes increasingly unresponsive over time, ultimately preventing any meaningful email delivery operations.
From a cybersecurity perspective, this vulnerability aligns with several ATT&CK framework techniques including T1499 for network denial of service and T1595 for network infiltration through service exploitation. The flaw also relates to CWE-400 which describes unspecified errors in resource management and CWE-691 which covers insufficient control flow management. Organizations affected by this vulnerability would experience service interruptions that could impact business operations, particularly in environments where email communication is critical for business processes. The vulnerability demonstrates the importance of implementing proper retry mechanisms with appropriate backoff strategies and maximum retry limits as recommended in industry best practices for network service resilience. Mitigation strategies would involve implementing proper rate limiting, configuring maximum retry attempts, and establishing appropriate error handling protocols that prevent the service from becoming trapped in infinite retry loops while maintaining legitimate delivery attempts for truly transient network issues.