CVE-1999-0638 in Host
Summary
by MITRE
The daytime service is running.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 04/18/2026
The CVE-1999-0638 vulnerability represents a fundamental security misconfiguration where the daytime service remains active on network systems. This service, traditionally designed to provide the current date and time to clients, operates on port 13 using the TCP/IP protocol stack. The vulnerability arises when this service is left enabled on systems without proper access controls or security considerations, creating an unnecessary attack surface that violates the principle of least privilege. The daytime service, while seemingly benign, provides attackers with basic system information that can aid in reconnaissance activities and system enumeration. This misconfiguration aligns with CWE-668, which addresses the exposure of a resource to the wrong sphere, and represents a classic example of insufficient system hardening practices that were prevalent in the late 1990s era of network security.
The technical flaw in CVE-1999-0638 stems from the default installation and configuration of operating systems and network services. The daytime service typically runs as a daemon process that listens for incoming connections on TCP port 13 and responds with the current date and time in a simple text format. When this service operates without proper authentication mechanisms or access controls, it becomes a readily available information disclosure vector. Attackers can connect to this service using simple telnet or netcat commands to retrieve system time information, which can be used for various malicious purposes including system fingerprinting and timing attacks. The vulnerability exists because the service operates with minimal security considerations, allowing any remote user to establish a connection and obtain basic time information without requiring any credentials or authorization.
The operational impact of CVE-1999-0638 extends beyond simple information disclosure, as it contributes to overall system security degradation and can facilitate more sophisticated attacks. While the daytime service itself does not directly enable privilege escalation or data theft, its presence increases the attack surface and provides attackers with additional reconnaissance capabilities. The information obtained through this service can be used to establish baseline system characteristics, which may be valuable for timing attacks against other services or for correlating events across multiple systems. This vulnerability demonstrates how seemingly harmless services can contribute to security weaknesses when improperly configured, and it represents a common pattern in early internet security where default configurations often prioritized functionality over security. The impact is particularly concerning in environments where proper network segmentation and access controls are not implemented, as it can serve as an entry point for more comprehensive reconnaissance efforts.
The recommended mitigations for CVE-1999-0638 focus on proper service management and system hardening practices that align with established security frameworks. Organizations should disable unnecessary services such as the daytime service through proper configuration management and system administration procedures. This approach directly addresses the vulnerability by removing the attack surface entirely, which corresponds to the ATT&CK technique T1562.001 for disabling or removing system protection. System administrators should implement regular security audits to identify and disable unused services, and employ network segmentation to limit access to critical systems. Additionally, organizations should configure firewalls to block access to port 13 from external networks and implement proper access controls that restrict who can connect to this service if it must remain active for legitimate purposes. The solution emphasizes the importance of principle of least privilege and demonstrates how basic security hygiene can effectively address this vulnerability category.