An industry organization has implemented a system to allow trusted authentication between all of its partners. The system consists of a web of trusted RADIUS servers communicating over the Internet. An attacker was able to set up a malicious server and conduct a successful man-in-the-middle attack. Which of the following controls should be implemented to mitigate the attack in the future?
A. Use PAP for secondary authentication on each RADIUS server
B. Disable unused EAP methods on each RADIUS server
C. Enforce TLS connections between RADIUS servers
D. Use a shared secret for each pair of RADIUS servers