Microsoft Publishes Security Advisory to Address MS-CHAPv2 Exploit

In response to the code required to crack MS-CHAPv2 sessions being published to the Web, Microsoft has issued a security advisory for organizations that may need guidance. However, the software giant has made it clear that while the issue is worth addressing, it is not something they plan to address with a security update.

MS-CHAP v2 is a widely used protocol, often serving as the primary authentication method in Point-to-Point Tunneling Protocol (PPTP)-based VPNs. During DEF CON, famed security researcher and cryptography guru Moxie Marlinspike released the tools and instructions needed to crack sessions using MS-CHAPv2. As noted in SecurityWeek’s coverage of his talk, most enterprises use MS-CHAPv2 for their PPTP VPNs, and WPA2-Enterprise traffic.

The first tool Marlinspike discussed was ChapCrack. After a packet capture is obtained with MS-CHAPv2 handshakes in it, ChapCrach can be used to parse the relevant credentials. These credentials are actually a single DES key, which can be sent to the second tool – CloudCracker. Once CloudCracker delivers, and it will, the output can be combined with ChapCrack and the entire packet capture is decrypted. 

“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)…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,” the security advisory explains.

Microsoft’s advisory is here, and additional information on MS-CHAPv2 is here.