Access Control List (ACL)

An Access Control↗ List (ACL) is a list of rules attached to a resource, a file, a network device, a router, that specifies exactly which users, groups, or systems are permitted or denied specific types of access to it. Each entry typically defines a subject, the resource, and the allowed action, like read, write, or execute.

ACLs are widely used both at the file-system level, controlling who can open or edit a document, and at the network level, controlling which traffic a firewall↗ or router allows through. They give administrators fine-grained control but can become difficult to manage at scale without regular review and cleanup.