RADIUS

The Access-Request Packet

There is no verification or authentication of the RADIUS Access-Request packet, as per the RFC specification, by default. The RADIUS server will perform a check to ensure that the message originated from an IP address listed as one of its clients, but in this day and age, spoofed IP addresses are easy to find and use. This is a serious limitation of the RADIUS protocol design.

As of now, the only workable solution is to require the presence of the Message-Authenticator attribute in all Access-Request messages. Briefly, the Message-Authenticator is the MD5 hash of the entire Access-Request message, using the client’s shared secret as the key. When a RADIUS server is configured to only accept Access-Request messages with a valid Message-Authenticator attribute present, it must silently discard those packets with invalid or missing attributes. More information on the Message-Authenticator attribute can be found in Chapter 9 or in the RFC 2869.

If your implementation somehow prevents the use of the Message-Authenticator attribute, at least consider using some sort of account-lockout feature, which disables authentications after a specified number of authentication attempts within a specified time.