https://www.cloudcracker.com/blog/2012/07/29/cracking-ms-chap-v2/
Very interesting read. . boleh has already mentioned the security risks in pptp on http://www.bolehvpn.net/pptp.php
Peace.
What Now?
1) All users and providers of PPTP VPN solutions should immediately start migrating to a different VPN protocol. PPTP traffic should be considered unencrypted.