On Monday August 20, Microsoft issued a warning about vulnerability in MS-CHAP v2 which could allow attackers to steal passwords from some wireless networks and VPNs. Only VPN solutions that rely on PPTP in combination with MS-CHAP v2 as the sole authentication method are vulnerable to this issue. Microsoft is aware that detailed exploit code has been published for known weaknesses in the Microsoft Challenge Handshake Authentication Protocol version 2 (MS-CHAP v2). The MS-CHAP v2 protocol is widely used as an authentication method in Point-to-Point Tunneling Protocol (PPTP)-based VPNs. Microsoft is not currently aware of active attacks that use this exploit code or of customer impact at this time. Microsoft is actively monitoring this situation to keep customers informed and to provide customer guidance as necessary.
Read Microsoft's Security Advisory here - http://technet.microsoft.com/en-us/security/advisory/2743314