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Access denial is a security tool used to limit access by persons who may be unauthorized. It uses an automated algorithm to determine if a user has permission to access a given resource or area. If the user does not, the system issues a warning about the access denial. The user may be able to enter additional verifying information or credentials to gain access.
A classic example of this security measure can be seen at secure sign in forms on the Internet. If a username is not known or someone enters the wrong password, the system uses access denial to prevent access to user accounts. It may provide a notice explaining what happened and inviting the user to try again, and will usually lock users out after a set number of failed tries for security reasons, to prevent attempts to brute-force a password.
Access denial can also be seen with an operating system, which limits administrative access for security reasons. Users who attempt to enter secured areas in a computer system will encounter errors. Likewise if they attempt to change settings that are beyond their permissions levels. Sometimes this is the result of an error; for example, a system may mistakenly refuse to allow a user to access an external hard drive, and an administrator may need to troubleshoot to determine the source of the problem.
It is also possible to use access denial in physical security, with pass codes, key cards, and other identifiers that keep unauthorized personnel out of secure areas. Posting a guard is also a form of access denial; the guard evaluates people who approach to determine if they should be allowed in, and can exclude them if they do not meet the parameters set by the parent organization. In physical security, access denial can also include the use of automatic locking gates and doors to limit movements through secured areas.
Systems that provide access denial services can operate at various levels of complexity. Basic security systems can reject incorrect passwords or identifiers, while others can send messages to administrators or alert law enforcement about attempts to tamper with physical facilities. This is only one aspect of security, which can also include encryption, internal password-protected areas, and other measures to control access and movement. If someone manages to penetrate an external security barrier, access to sensitive internal areas may be limited by layers of security.