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Figure 1: NIST SP 800-162 ABAC Definition
ABAC Benefits
The key advantage of ABAC is that it does not allow application developers to hardcode a static list of roles and oversimplify their authorization source code. Rather, ABAC forces them to centralize all authorization decisions and make runtime calls for a decision based on the Subject, Resource, Action, and Environment request attributes. Another key benefit is the simplicity of ABAC, which can make it easier to understand how a rule grants access to a resource. In contrast, RBAC does seem foreign to many users, and, especially during the early phase of its adoption, the levels of abstraction can be challenging for an IT team.
One other added advantage of ABAC is its flexibility. With ABAC, as long as the necessary data is available, almost anything can be represented as a rule-based query. For example, a rule evaluated at runtime in a login session can use contextual information—even information passed in via SAML claims or JWT tokens. In contrast, when delivering the role membership for a user to the application, a standard RBAC engine would not evaluate this type of information.
ABAC Primary Benefits
ABAC enforces centralized management of authorization policies.
ABAC makes it easy to specify access rules as simple queries.
ABAC rules can be extraordinarily fine-grained and contextual.
ABAC rules can evaluate attributes of Subjects and Resources that are not inventoried by the authorization system.
ABAC rules need less maintenance and overhead because they do not require the creation or maintenance of the structure on which an RBAC model depends, e.g., roles and resource locations.
Figure 2 , below, shows an example of an actual ABAC policy implemented in an EmpowerID demonstration. Here, our application is asking if Alice can View Bob’s X-Ray. Our ABAC decision engine will base its decision on an elaborate ABAC policy (labeled Policies in the figure). The first policy check, Rule1, is that the action being taken is ‘View.’ The next extended requirement for Rule1 is that the company is not in ‘Emergency Mode,’i.e., in the middle of a crisis. However, to further extend the rule flexibility during a crisis, some roles may be granted additional permissions to enable those who would typically not be permitted to perform those activities to assist. The next check is only to allow ‘View ‘if the person is currently on the ‘Local Network.’ (Some sensitive activities might not be allowed if the person is outside the corporate network.) The next check confirms that the user performed a Multi-Factor Authentication (‘MFA’) with a Level of Assurance (‘LOA’) of at least ‘2’. This type of authentication refers to a stronger authentication method, like a physical token or mobile push. The policy also checks to see if the user is a member of the Doctors role and that their status is not currently set to out of office. Finally, the checks ensure that the X-Ray has neither been flagged as ‘Confidential’ nor the Subject has a relationship of the type Attending Physician.
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Figure 2: Elaborate ABAC policy showing the potential of ABAC
ABAC Weaknesses
The first challenge encountered with implementing policies like this is the information needs to be obtained and evaluated at runtime. In the above policy example, if we needed to know if the user’s nationality was Swiss, then this information would likely reside in either the corporate Active Directory or HR system (Figure 3). Moreover, if we needed to know if the company was in Emergency Mode or not, this essential information might be difficult to obtain in a live corporate environment. Likewise, if we needed to ascertain their out of office status, this would reside in a corporate email system such as Office 365. Furthermore, to attain information for network login sessions or the MFA status would require a query to an Identity Provider such as Ping Federate.
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Figure 4: A Common Standard Auditor's Question
ABAC’s Primary Weaknesses
ABAC makes it extremely difficult to perform a “before the fact audit” and determine the permissions available to a specific user. To successfully determine access, not only might a considerable number of rules need to be executed, but they must also be done in the same order in which the system applies them. As a result, this could make it impossible to assess risk exposure for any given employee position.
In a comparable manner to how a “Role Explosion” can occur with RBAC, an explosion can also occur with ABAC where a system with N number of attributes would have 2N possible rule combinations.
Unless rules are kept extremely simple and do not access data from various source systems, ABAC systems with complex rules from multiple attribute sources can be unacceptably slow to answer authorization queries.
ABAC Challenges
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Figure 5: The Who, What, and How of ABAC Challenges
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Another challenge with ABAC is finding someone within the organization with the skills and knowledge to write authorization policies. To write rich, accurate, and effective policies requires both solid business knowledge and a deep understanding of security. XACML was developed 18 years ago as a standards-based language built on XML to write authorization policies. The original idea with XACML was that business users would author these policies and, in vendor marketing and some analyst circles, this idea persists to this day. The notion that business users would know best how to determine who should be able to do what from a business activity perspective is, after all, entirely logical. However, writing XACML policies is extremely complex. and most business users simply do not possess the time to write IT policies.
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See Also
What is Role Based Access Control?
What is Policy Based Access Control?
EmpowerID Hybrid Access Control (RBAC, ABAC, and PBAC)