3.3: Access Control - Models

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Access Control Models

Access control models are sometimes categorized as either discretionary or non-discretionary. The three most widely recognized models are Discretionary Access Control (DAC), Mandatory Access Control (MAC), and Role Based Access Control (RBAC). MAC is non-discretionary. There are additional access control system but they are not as widely deployed as the top three.

Discretionary access control

Discretionary access control (DAC) is a policy determined by the owner of an object. The owner decides who is allowed to access the object, and what privileges they have.

Two important concepts in DAC are

File and data ownership: Every object in the system has an owner. In most DAC systems, each object's initial owner is the subject that caused it to be created. The access policy for an object is determined by its owner.
Access rights and permissions: These are the controls that an owner can assign to other subjects for specific resources.

Access controls may be discretionary in ACL-based or capability-based access control systems. (In capability-based systems, there is usually no explicit concept of 'owner', but the creator of an object has a similar degree of control over its access policy.)

Mandatory access control

Mandatory access control refers to allowing access to a resource if and only if rules exist that allow a given user to access the resource. It is difficult to manage, but its use is usually justified when used to protect highly sensitive information. Examples include certain government and military information. Management is often simplified (over what is required) if the information can be protected using hierarchical access control, or by implementing sensitivity labels. What makes the method "mandatory" is the use of either rules or sensitivity labels.

Sensitivity labels: In such a system subjects and objects must have labels assigned to them. A subject's sensitivity label specifies its level of trust. An object's sensitivity label specifies the level of trust required for access. In order to access a given object, the subject must have a sensitivity level equal to or higher than the requested object.
Data import and export: Controlling the import of information from other systems and export to other systems (including printers) is a critical function of these systems, which must ensure that sensitivity labels are properly maintained and implemented so that sensitive information is appropriately protected at all times.

Two methods are commonly used for applying mandatory access control:

Rule-based (or label-based) access control: This type of control further defines specific conditions for access to a requested object. A Mandatory Access Control system implements a simple form of rule-based access control to determine whether access should be granted or denied by matching:
- An object's sensitivity label
- A subject's sensitivity label
Lattice-based access control: These can be used for complex access control decisions involving multiple objects and/or subjects. A lattice model is a mathematical structure that defines greatest lower-bound and least upper-bound values for a pair of elements, such as a subject and an object.

Few systems implement MAC; systems based on the operating systems XTS-400 and SELinux are examples of systems that do.

Role-based access control

Role-based access control (RBAC) is an access policy determined by the system, not by the owner. RBAC is used in commercial applications and also in military systems, where multi-level security requirements may also exist. RBAC differs from DAC in that DAC allows users to control access to their resources, while in RBAC, access is controlled at the system level, outside of the user's control. Although RBAC is non-discretionary, it can be distinguished from MAC primarily in the way permissions are handled. MAC controls read and write permissions based on a user's clearance level and additional labels. RBAC controls collections of permissions that may include complex operations such as an e-commerce transaction, or may be as simple as read or write. A role in RBAC can be viewed as a set of permissions.

Three primary rules are defined for RBAC:

Role assignment: A subject can execute a transaction only if the subject has selected or been assigned a suitable role.
Role authorization: A subject's active role must be authorized for the subject. With rule 1 above, this rule ensures that users can take on only roles for which they are authorized.
Transaction authorization: A subject can execute a transaction only if the transaction is authorized for the subject's active role. With rules 1 and 2, this rule ensures that users can execute only transactions for which they are authorized.

Additional constraints may be applied as well, and roles can be combined in a hierarchy where higher-level roles subsume permissions owned by lower-level sub-roles.

Most IT vendors offer RBAC in one or more products.

Attribute-based access control

In attribute-based access control (ABAC), access is granted not based on the rights of the subject associated with a user after authentication, but based on the attributes of the user. The user has to prove so-called claims about his or her attributes to the access control engine. An attribute-based access control policy specifies which claims need to be satisfied in order to grant access to an object. For instance the claim could be "older than 18". Any user that can prove this claim is granted access. Users can be anonymous when authentication and identification are not strictly required. One does, however, require means for proving claims anonymously. This can for instance be achieved using anonymous credentials. XACML (extensible access control markup language) is a standard for attribute-based access control. XACML 3.0 was standardized in January 2013.

Break-Glass Access Control Models

Traditionally, access has the purpose of restricting access, thus most access control models follow the "default deny principle", i.e. if a specific access request is not explicitly allowed, it will be denied. This behavior might conflict with the regular operations of a system. In certain situations, humans are willing to take the risk that might be involved in violating an access control policy, if the potential benefit that can be achieved outweighs this risk. This need is especially visible in the health-care domain, where a denied access to patient records can cause the death of a patient. Break-Glass (also called break-the-glass) try to mitigate this by allowing users to override access control decision. Break-Glass can either be implemented in an access control specific manner (e.g. into RBAC), or generic (i.e., independent from the underlying access control model).

Adapted from:
"Computer access control" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0