A system and method provide comprehensive and highly automated testing of vulnerabilities to intrusion on a target network, including identification of operating system, identification of target network topology and target computers, identification of open target ports, assessment of vulnerabilities on target ports, active assessment of vulnerabilities based on information acquired from target computers, quantitative assessment of target network security and vulnerability, and hierarchical graphical representation of the target network, target computers, and vulnerabilities in a test report. The system and method employ minimally obtrusive techniques to avoid interference with or damage to the target network during or after testing.