Assessment of access control systems

Abstract

Adequate security of information and information systems is a fundamental management responsibility. Nearly all applications that deal with financial, privacy, safety, or defense include some form of access control. Access control is concerned with determining the allowed activities of legitimate users, mediating every attempt by a user to access a resource in the system. In some systems, complete access is granted after successful authentication of the user, but most systems require more sophisticated and complex control. In addition to the authentication mechanism (such as a password), access control is concerned with how authorizations are structured. In some cases, authorization may mirror the structure of the organization, while in others it may be based on the sensitivity level of various documents and the clearance level of the user accessing those documents. This publication explains some of the commonly used access control services available in information technology systems. Organizations planning to implement an access control system should consider three abstractions: access control policies, models, and mechanisms. Access control policies are highlevel requirements that specify how access is managed and who may access information under what circumstances. For instance, policies may pertain to resource usage within or across organizational units or may be based on need-to-know, competence, authority, obligation, or conflict-of-interest factors. At a high level, access control policies are enforced through a mechanism that translates a user’s access request, often in terms of a structure that a system provides. An access control list is a familiar example of an access control mechanism. Access control models bridge the gap in abstraction between policy and mechanism. Rather than attempting to evaluate and analyze access control systems exclusively at the mechanism level, security models are usually written to describe the security properties of an access control system. Security models are formal presentations of the security policy enforced by the system and are useful for proving theoretical limitations of a system. Discretionary access control, which allows the creator of a file to delegate access to others, is one of the simplest examples of a model. As systems grow in size and complexity, access control is a special concern for systems that are distributed across multiple computers. These distributed systems can be a formidable challenge for developers, because they may use a variety of access control mechanisms that must be integrated to support the organization’s policy; for example, role-based access control that can enforce administrator-specified rules is often used. Popular database management system designs, such as Structured Query Language (SQL), incorporate many aspects of roleand rulebased access. Services that are particularly useful in implementing distributed access control include the Lightweight Directory Access Protocol (LDAP), capability-based Kerberos, and the Extensible Markup Language (XML)-based Extensible Access Control Markup Language (XACML). A state of access control is said to be safe if no permission can be leaked to an unauthorized or uninvited principal. To assure the safety of an access control system, it is essential to make certain that the access control configuration (e.g., access control model) will not result in the leakage of permissions to an unauthorized principal. Even though the general safety computation