Short for Remote Authentication Dial-In User Service, RADIUS harkens back to the days of dial-in Internet and network access through public switched telephone networks. It has remained the de facto standard for lightweight authentication ever since and is supported in virtually all switches, routers, access points, and VPN concentrators shipped in the past two decades.

Source: New Blast-RADIUS attack breaks 30-year-old protocol used in networks everywhere | Ars Technica