A host-based intrusion detection system (HIDS) sensor that monitors system logs for evidence of malicious or suspicious application activity running in real time and monitors key system files for evidence of tampering. This system detects attacks targeted at the host system on which it is installed and monitors output to the system and audit logs. It is signature-based and identifies and analyzes system and audit messages for signs of system misuse or attack. The system monitors the logs of applications running on the host, including mail servers, web servers and FTP servers. The system also monitors system files and notifies the system administrator when key system and security files have been accessed, modified or even deleted.