Microsoft pptp vpn vulnerabilities Exploits in Action

Signature of the attack:
Since the attack uses a “sniffer” technique for this variation of the attack, 
the only warning you might have is if your IDS, or a manual audit, detects a 
system with it’s NIC in “promiscuous mode”. That would be the attacking 
system.
Any users (MS-CHAP version 1) complaining of password reset requests could 
also be a warning sign.
How to protect against it:
Do not use any client or server that use MS-CHAP version 1.
Be sure to update all clients and servers and follow MS recommendations. 
Unfortunately, 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.