Access Control, also known as Authorization — is mediating access to resources on the basis of identity and is generally policy-driven (although the policy may be implicit). It is the primary security service that concerns most software, with most of the other security services supporting it. For example, access control decisions are generally enforced on the basis of a user-specific policy, and authentication is the way to establish the user in question. Similarly, confidentiality is really a manifestation of access control, specifically the ability to read data. Since, in computer security, confidentiality is often synonymous with encryption, it becomes a technique for enforcing an access-control policy.
Policies that are to be enforced by an access-control mechanism generally operate on sets of resources; the policy may differ for individual actions that may be performed on those resources (capabilities). For example, common capabilities for a file on a file system are: read, write, execute, create, and delete. However, there are other operations that could be considered “meta-operations” that are often overlooked — particularly reading and writing file attributes, setting file ownership, and establishing access control policy to any of these operations.
Often, resources are overlooked when implementing access control systems. For example, buffer overflows are a failure in enforcing write-access on specific areas of memory. Often, a buffer overflow exploit also accesses the CPU in a manner that is implicitly unauthorized as well.
Advantage of Mandatory Access Control
From the perspective of end-users of a system, access control should be mandatory whenever possible, as opposed to discretionary. Mandatory access control means that the system establishes and enforces a policy for user data, and the user does not get to make his own decisions of who else in the system can access data. In discretionary access control, the user can make such decisions. Enforcing a conservative mandatory access control policy can help prevent operational security errors, where the end user does not understand the implications of granting particular privileges. It usually keeps the system simpler as well.
Mandatory access control is also worth considering at the OS level, where the OS labels data going into an application and enforces an externally defined access control policy whenever the application attempts to access system resources. While such technologies are only applicable in a few environments, they are particularly useful as a compartmentalization mechanism, since — if a particular application gets compromised — a good MAC system will prevent it from doing much damage to other applications running on the same machine.