RADIUS (Remote Authentication Dial-In User Service) is a protocol used to centralize authentication, authorization, and accounting for network access. Devices such as VPN gateways, wireless controllers, and switches send login requests to a RADIUS server, which decides whether a user or device may connect and what level of access is allowed.
In cyber security, RADIUS is important because it often sits on the trust boundary between the internet edge and internal systems. If attackers reach a RADIUS server or its database, they may learn which accounts are valid, reuse stolen credentials, or try to weaken access controls. Defenders protect it by limiting exposure, using strong encryption between clients and servers, monitoring failed logins and unusual policy changes, and separating identity infrastructure from perimeter devices. In incidents, attackers may target RADIUS not just for passwords, but to control the systems that grant access in the first place.