CVE-2000-0990 in cmd5checkpw
Summary
by MITRE
cmd5checkpw 0.21 and earlier allows remote attackers to cause a denial of service via an "SMTP AUTH" command with an unknown username.
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Analysis
by VulDB Data Team • 05/28/2018
The vulnerability identified as CVE-2000-0990 affects cmd5checkpw version 0.21 and earlier, representing a significant security weakness in email authentication systems. This flaw resides in the handling of SMTP AUTH commands within the command-line tool designed for password verification in email servers. The vulnerability manifests when the system receives an SMTP AUTH command containing an unknown username, leading to a denial of service condition that can disrupt legitimate email services.
The technical implementation of this vulnerability stems from inadequate input validation and error handling within the cmd5checkpw utility. When processing SMTP AUTH requests, the software fails to properly manage unknown user accounts, causing the application to crash or become unresponsive. This occurs because the program does not implement proper exception handling for invalid authentication attempts, particularly when encountering usernames that do not exist in the system's user database. The flaw represents a classic case of improper error handling that can be exploited through crafted malicious input to cause system instability.
From an operational perspective, this vulnerability poses a substantial risk to email server availability and reliability. Attackers can exploit this weakness by sending carefully crafted SMTP AUTH commands with non-existent usernames, effectively triggering the denial of service condition. This type of attack can be executed with minimal resources and technical expertise, making it particularly dangerous for systems that rely on cmd5checkpw for email authentication. The impact extends beyond simple service disruption, as it can potentially be used as part of larger attack campaigns to compromise email infrastructure availability.
The vulnerability aligns with CWE-20, which describes "Improper Input Validation" and CWE-400, which covers "Uncontrolled Resource Consumption." These classifications reflect the core issues within the cmd5checkpw implementation where insufficient validation leads to resource exhaustion and system instability. The attack vector follows patterns consistent with the MITRE ATT&CK framework's T1499.004 technique, specifically "Network Denial of Service" targeting email services. Organizations using vulnerable versions of cmd5checkpw should consider implementing network-level protections and monitoring for unusual SMTP AUTH patterns as part of their defensive strategies.
Mitigation strategies should include immediate patching of cmd5checkpw to version 0.22 or later, which contains the necessary fixes for proper error handling. Additionally, administrators should implement rate limiting on SMTP AUTH commands to prevent abuse, deploy intrusion detection systems to monitor for suspicious authentication patterns, and consider implementing proper logging and alerting mechanisms to detect exploitation attempts. Network segmentation and firewall rules can also help limit the attack surface by restricting access to vulnerable email services. The vulnerability underscores the importance of robust input validation and error handling in security-critical applications, particularly those handling authentication requests in network services.