Microsoft pptp vpn vulnerabilities Exploits in Action

Signature of the attack:
The same as Attack #4, a NIC in promiscuous mode could be a warning 
sign. Of course, any users (MS-CHAP version 1) complaining of password reset 
requests could also be a warning sign.
How to protect against it:
Though the improved versions of MS-CHAP and PPTP make 
compromising the hashes slightly more difficult when using Version 2 instead of 
1, it is still far too simple to compromise this information. Strong passwords that 
are not dictionary based will be much more resistant to such attacks, if even one 
user password is dictionary based, then it is likely an attacker will crack it in 
time, and possibly fast enough to compromise the infrastructure.
Also, according to several resources (Counterpane Labs MS PPTP Version 2 
article Section 5.1 “Version Rollback Attacks”
http://www.counterpane.com/pptpv2-paper.html
) even following the MS 
recommendations, the server and clients can still be fooled into downgrading to 
MS-CHAP version 1.
One option, if using PPTP, is not to use any MS product for the PPTP 
client or server. The Linux and other *Nix & *BSD variants allow more control of 
the PPTP client and server, and can be kept from doing version rollbacks from 
MS Chap V2 to V1, this combined with very strong passwords appears to be a  
more robust solution, though most companies that are “Windows shops” are not 
likely to take this approach.
The best option would be to migrate away from PPTP to one of the other 
protocols such as IPSec.