Which access control model is considered the most restrictive?

Mandatory Access Control (MAC) is recognized as the most restrictive access control model because it enforces strict policies determined by the system as opposed to individual users. In this model, resources are classified and users are granted access based on their security clearances and the classification of the information, leading to a high level of control over who can access what. This means that users cannot change access permissions or share information freely; all access is mediated by the system following set policies.

In contrast, models like Discretionary Access Control (DAC) allow users to have more control over their own resources, enabling them to determine who can access those resources. Role-Based Access Control (RBAC) relies on a user's assigned roles to define access, which can be flexible. Attribute-Based Access Control (ABAC) utilizes attributes for access decisions but still allows for a level of discretion that could be less restrictive than MAC.

Therefore, the inherent structure of MAC, with its centralized control and emphasis on security clearances, establishes it as the most stringent access control model available.