This essay provides an overview of the vulnerabilities and threats to information security in computer systems. It begins with a historical presentation of past experiences with vulnerabilities in communication security along with present and future computer security experiences. The historical perspective demonstrates that misplaced confidence in the security of a system is worse than having no confidence at all in its security. Next, the essay describes four broad areas of computer misuse: (1) theft of computational resources, (2) disruption of computational services, (3) unauthorized disclosure of information in a computer, and (4) unauthorized modification of information in a computer. Classes of techniques whereby computer misuse results in the unauthorized disclosure and modification of information are then described and examples are provided. These classes are (1) human error, (2) user abuse of authority, (3) direct probing, (4) probing with malicious software, (5) direct penetration, and (6) subversion of security mechanism. The roles of Trojan horses, viruses, worms, bombs, and other kinds of malicious software are described and examples provided.