Skip to main content

Thirty Years Later: Lessons from the Multics Security Evaluation

This is the Revised Version.

This Research Report consists of two invited papers for the Classic Papers section of the 18th Annual Computer Security Applications Conference (ACSAC) to be held 9-13 December 2002 in Las Vegas, NV. The papers will be available on the web after the conference at

The first paper, Thirty Years Later: Lessons from the Multics Security Evaluation, is a commentary on the second paper, discussing the implications of the second paper’s results on contemporary computer security issues. Copyright will be transferred on the first paper.

The second paper, Multics Security Evaluation: Vulnerability Analysis is a reprint of a US Air Force report, first published in 1974. It is a government document, approved for public release, distribution unlimited, and is not subject to copyright. This reprint does not include the original computer listings. They can be found at

Abstract of Thirty Years Later: Lessons from the Multics Security Evaluation

Almost thirty years ago a vulnerability assessment of Multics identified significant vulnerabilities, despite the fact that Multics was more secure than other contemporary (and current) computer systems. Considerably more important than any of the individual design and implementation flaws was the demonstration of subversion of the protection mechanism using malicious software (e.g., trap doors and Trojan horses). A series of enhancements were suggested that enabled Multics to serve in a relatively benign environment. These included addition of "Mandatory Access Controls" and these enhancements were greatly enabled by the fact the Multics was designed from the start for security. However, the bottom-line conclusion was that "restructuring is essential" around a verifiable "security kernel" before using Multics (or any other system) in an open environment (as in today's Internet) with well-motivated professional attacks employing subversion. The lessons learned from the vulnerability assessment are highly applicable today as governments and industry strive (unsuccessfully) to "secure" today's weaker operating systems through add-ons, "hardening", and intrusion detection schemes.

Abstract of Multics Security Evaluation: Vulnerability Analysis

A security evaluation of Multics for potential use as a two-level (Secret/Top Secret) system in the Air Force Data Services Center (AFDSC) is presented. An overview is provided of the present implementation of the Multics Security controls. The reports then details the results of a penetration exercise of Multics on the HIS 645 computer. In addition, preliminary results of a penetration excise of Multics on the new HIS 6180 computer are presented. The report concludes that Multics as implemented today is not certifiably secure and cannot be used in an open use multi-level system. However, the Multics security design principles are significantly better than other contemporary systems. Thus, Multics as implemented today, can be used in a benign Secret/Top Secret environment. In addition, Multics forms a base from which a certifiably secure open use multi-level system can be developed.

By: Paul A. Karger, Roger R. Schell

Published in: Proceedings 18th Annual Computer Security Applications Conference. Los Alamitos, CA, , IEEE Computer Society. , p.119-26 in 2002

Please obtain a copy of this paper from your local library. IBM cannot distribute this paper externally.

Questions about this service can be mailed to .