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Access control list (ACL) refers to the permissions attached to an object that specify which users are granted access to that object and the operations it is allowed to perform.
Each entry in an access control list specifies the subject and an associated operation that is permitted.
File system ACL is a data structure that holds entries that specify individual user or group rights to system objects such as processes, files and programs. These entries are referred to as access control entities. Each system object is associated with a security attribute that identifies its access control list.
The ACL has an entry for each system user that defines the user's privileges, such as reading a file, writing to a file or executing a file. The operating systems that use ACL include Novell’s Netware, Microsoft Windows NT/2000, Digital's OpenVMS and UNIX-based systems.
When a subject requests an object in an ACL-based security model, the OS initially checks the ACL for an applicable entry to decide whether the requested operation is authorized. The ACL model is applicable to both individual entities and the collection of objects within the system hierarchy.