Secure your RDP servers with MultiDeskEnforcer or Guacamole

MultiDeskEnforcer Adds the requirement of a shared key in addition to the usual username/password to establish an RDP connection.

http://www.syvik.com/multidesk/enforcer.en.html

Open source Guacamole is a much more comprehensive solution which can force 2FA or TOTP in addition to username/password.  It also allows clients to connect using only a browser!

https://guacamole.apache.org/