THIS REPORT IS APPLICABLE TO MOST GENERAL PURPOSE COMPUTER OPERATING SYSTEMS; HOWEVER, IT DISCUSSES, IN DETAIL, THE SECURITY FEATURES OF THREE OPERATING SYSTEMS. THESE SYSTEMS ARE: IB7S OS/MVT, UNIVAC'S 1100 SERIES OPERATING SYSTEM, AND BOLT BERANEK AND NEWMAN'S TENEX SYSTEM FOR THE PDP-10. THEY WERE CHOSEN FOR THEIR ILLUSTRATIVE VALUE--TWO OF THEM BECAUSE THEY ARE THE MOST COMMONLY USED LARGE SYSTEMS IN THE FEDERAL GOVERNMENT INVENTORY, AND THE THIRD BECAUSE A DETAILED ANALYSIS OF ITS SECURITY WAS AVAILABLE, AND BECAUSE MANY OF THE SPECIFIC SECURITY FLAWS FOUND IN THE SYSTEM CAN BE USED AS DETAILED EXAMPLES OF TYPICAL SECURITY FLAWS. BECAUSE OF THE SIMILARITY OF OPERATING SYSTEMS AND THEIR SECURITY PROBLEMS, SPECIFIC SECURITY FLAWS ARE FORMALLY CLASSIFIED ACCORDING TO A TAXONOMY DEVELOPED IN THE REPORT. THIS CLASSIFICATION LEADS TO A CLEARER UNDERSTANDING OF SECURITY FLAWS AND AIDS IN ANALYZING NEW SYSTEMS. THE DISCUSSIONS OF SECURITY FLAWS AND THE SECURITY ENHANCEMENTS OFFER A STARTING REFERENCE FOR PLANNING A SECURITY INVESTIGATION OF AN EDP (ELECTRONIC DATA PROCESSING) INSTALLATION'S OPERATING SYSTEM. (AUTHOR ABSTRACT MODIFIED)....EB