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Access control

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Access control is the ability to permit or deny the use of something by someone.


Physical access

Physical access of a person may be allowed depending on payment, authorization, etc. Also there may be one-way traffic of people. These can be enforced by personnel such as a border guard, a doorman, a ticket checker, etc., or with a device such as a turnstile. There may be fences to avoid circumventing this access control. An alternative of access control in the strict sense (physically controlling access itself) is a system of checking authorized presence, see e.g. Ticket controller. A variant is exit control, e.g. of a shop (checkout) or a country.

In physical security, the term access control refers to the practice of restricting entrance to a property, a building, or a room to authorized persons. Physical access control can be achieved by a human (a guard, bouncer, or receptionist), through mechanical means such as locks and keys, or through technological means such as a card access system or biometric identification.

Computer security

In computer security, access control includes authentication, authorization and audit. It also includes measures such as physical devices, including biometric scans and metal locks, hidden paths, digital signatures, encryption, social barriers, and monitoring by humans and automated systems.

In any access control model, the entities that can perform actions in the system are called subjects, and the entities representing resources to which access may need to be controlled are called objects (see also Access Control Matrix). Subjects and objects should both be considered as software entities, rather than as human users: any human user can only have an effect on the system via the software entities that they control. Although some systems equate subjects with user IDs, so that all processes started by a user by default have the same authority, this level of control is not fine-grained enough to satisfy the Principle of least privilege, and arguably is responsible for the prevalence of malware in such systems (see computer insecurity).

In some models, for example the object-capability model, any software entity can potentially act as both a subject and object.

Access control models used by current systems tend to fall into one of two classes: those based on capabilities and those based on access control lists (ACLs). In a capability-based model, holding an unforgeable reference or capability to an object provides access to the object (roughly analogous to how possession of your house key grants you access to your house); access is conveyed to another party by transmitting such a capability over a secure channel. In an ACL-based model, a subject's access to an object depends on whether its identity is on a list associated with the object (roughly analogous to how a bouncer at a private party would check your ID to see if your name is on the guest list); access is conveyed by editing the list. (Different ACL systems have a variety of different conventions regarding who or what is responsible for editing the list and how it is edited.)

Both capability-based and ACL-based models have mechanisms to allow access rights to be granted to all members of a group of subjects (often the group is itself modeled as a subject).

Access control systems provide the essential services of identification and authentication (I&A), authorization, and accountability where:

  • identification and authentication determine who can log on to a system, and the association of users with the software subjects that they are able to control as a result of logging in;
  • authorization determines what a subject can do;
  • accountability identifies what a subject (or all subjects associated with a user) did.

Identification and authentication (I&A)

Identification and authentication (I&A) is a two-step process that determines who can log on to a system. Identification is how a user tells a system who he or she is (for example, by using a username). The identification component of an access control system is normally a relatively simple mechanism based on either Username or User ID. In the case of a system or process, identification is usually based on:

  • Computer name
  • Media Access Control (MAC) address
  • Internet Protocol (IP) address
  • Process ID (PID)

The only requirements for identification are that the identification:

  • Must uniquely identify the user.
  • Shouldn't identify that user's position or relative importance in an organization (such as labels like president or CEO).
  • Should avoid using common or shared user accounts, such as root, admin, and sysadmin. Such accounts provide no accountability and are juicy targets for hackers.

Authentication is the process of verifying a user's claimed identity (for example, by comparing an entered password to the password stored on a system for a given username).

Authentication is based on at least one of these four factors:

  • Something you know, such as a password or a personal identification number (PIN). This assumes that only the owner of the account knows the password or PIN needed to access the account.
  • Something you have, such as a smart card or token. This assumes that only the owner of the account has the necessary smart card or token needed to unlock the account.
  • Something you are, such as fingerprint, voice, retina, or iris characteristics.
  • Where you are, for example inside or outside a company firewall, or proximity of login location to a personal GPS device.


Authorization applies to subjects rather than to users (the association between a user and the subjects initially controlled by that user having been determined by I&A). Authorization determines what a subject can do on the system.

Most modern operating systems define sets of permissions that are variations or extensions of three basic types of access:

  • Read (R): The subject can
    • Read file contents
    • List directory contents
  • Write (W): The subject can change the contents of a file or directory with these tasks:
    • Add
    • Create
    • Delete
    • Rename
  • Execute (X): If the file is a program, the subject can cause the program to be run. (In Unix systems, the 'execute' permission doubles as a 'traverse directory' permission when granted for a directory.)

These rights and permissions are implemented differently in systems based on discretionary access control (DAC) and mandatory access control (MAC).


Accountability uses such system components as audit trails (records) and logs to associate a subject with its actions. The information recorded should be sufficient to map the subject to a controlling user. Audit trails and logs are important for

  • Detecting security violations
  • Re-creating security incidents

If no one is regularly reviewing your logs and they are not maintained in a secure and consistent manner, they may not be admissible as evidence.

Many systems can generate automated reports based on certain predefined criteria or thresholds, known as clipping levels. For example, a clipping level may be set to generate a report for the following:

  • More than three failed logon attempts in a given period
  • Any attempt to use a disabled user account

These reports help a system administrator or security administrator to more easily identify possible break-in attempts.

Access Control Techniques

Access control techniques are sometimes categorized as either discretionary or mandatory.

Discretionary Access Control

Discretionary access control (DAC) is an access 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, capability-based, or Role-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 (MAC) is an access policy determined by the system, not the owner. MAC is used in multilevel systems that process highly sensitive data, such as classified government and military information. A multilevel system is a single computer system that handles multiple classification levels between subjects and objects.

  • Sensitivity labels: In a MAC-based system, all 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 MAC-based 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 access controls: This type of control further defines specific conditions for access to a requested object. All MAC-based systems implement 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 controls: 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. XTS-400 is an example of one that does.


In telecommunication, the term access control is defined in U.S. Federal Standard 1037C [1] with the following meanings:

  1. A service feature or technique used to permit or deny use of the components of a communication system.
  2. A technique used to define or restrict the rights of individuals or application programs to obtain data from, or place data onto, a storage device.
  3. The definition or restriction of the rights of individuals or application programs to obtain data from, or place data into, a storage device.
  4. The process of limiting access to the resources of an AIS to authorized users, programs, processes, or other systems.
  5. That function performed by the resource controller that allocates system resources to satisfy user requests.

Notice that this definition depends on several other technical terms from Federal Standard 1037C.

Public Policy

In public policy, access control to restrict access to systems ("authorization") or to track or monitor behavior within systems ("accountability") is an implementation feature of using trusted systems for security or social control.

See Also


External links