Method and apparatus for protecting a data processing system such as an Internet server from attack by a vandal who uses an offensive vulnerability scanner to find an externally visible vulnerability of the data processing system. The method includes determining an externally visible vulnerability using a defensive vulnerability scanner, configuring an intrusion detection system to detect a network flow associated with the vulnerability, and blocking that flow by a firewall or a router. The apparatus includes a defensive vulnerability scanner that finds an externally visible vulnerability and provides a description of the vulnerability, an intrusion detection system that detects a network flow that satisfies the description, and a firewall or a router that blocks the flow responsive to detection of the flow by the intrusion detection system.