Microsoft pptp vpn vulnerabilities Exploits in Action

How to protect against it:
Same as Attack #1 description:
Download MS patch from 
http://www.microsoft.com/technet/treeview/default.asp?url=/technet/security/bull
etin/MS01-009.asp
or install service Pack 6a or migrate to Windows 2000.
Another possible work around is to try to filter GRE packets by their 
source address at your perimeter, only allowing traffic from known addresses.  
However, since GRE is a connectionless protocol, source address spoofing is 
trivial.  There are a number of tools and sites describing how to abuse any 
networks that allow any kind of GRE traffic. If an attacker can guess what 
source addresses are allowed, the attacker can simply send packets with the 
allowed source IP forged and bypass the filtering.