The author explains how he was caught tapping into GTE Telenet Corporations' Telemail electronic mail system, which serves security-conscious government and business clients. He also recounts his experiences after he was arrested. For system operators who know their systems are being penetrated, the book provides a step-by-step method for determining what the hacker has done and how to defeat such intrusion into the system. Signs of the teenage hacker syndrome are highlighted. To prevent hacker intrusions, designers of computer security systems are told to think like hackers so as to rid their systems of common security loopholes known to every hacker. Practical advice is offered for making computer systems tamperproof at minimal cost. The book also analyzes the cost effectiveness of external security devices and discusses necessary inhouse steps to protect valuable data. Particular attention is given to microcomputer security. The appendix contains an evaluation of some available security equipment. A subject index is provided.