Security in computer systems is important to ensure reliable operation and protect the integrity of stored information.  Faults in the implementation can be exploited to breach security and penetrate an operating system.  These faults must be identified, detected, and corrected to ensure reliability and safe-guard against denial of service, unauthorized modification of data, or disclosure of information. We define a classification of security faults in the Unix operating system.  We state the criteria used to categorize the faults and present examples of the different fault types. We present the design and implementation details of a database to store vulnerability information collected from different sources.  The data is organized according to our fault categories.  the information in the database can be applied in static audit analysis of systems, intrusion detection, and fault detection.  We also identify and describe software testing methods that should be effective in detecting different faults in our classification scheme.