Agentless Backup is Not a Myth

Security researcher Moxie Marlinspike has turned his attention to VPNs based on Microsoft’s MS-CHAPv2 protocol, demonstrating software at Defcon that can capture and crack passwords.

Chapcrack parses the credential information out of MS-CHAPv2 handshakes, which are then sent to Cloudcracker. Cloudcracker will then return a packet that can be decrypted by Chapcrack to recover the password.

Marlinspike writes in this blog post, “It shows up most notably in PPTP VPNs, and is also used quite heavily in WPA2 Enterprise environments”.

There is, he writes, “only one unknown in the entire protocol – the MD4 hash of the user’s passphrase, which is used to construct three separate DES keys”. Since the MD4 hash of a user’s password “is enough to authenticate as them”, Marlinspike and collaborator David Hulton of Pico computing made this the focus of Chapcrack.

While El Reg doesn’t propose reproducing Marlinspike’s technical explanation in full, it’s worth a read, if only for the reasoning behind how the attack works: from what looks like a seriously difficult computational task, he and Hulton winnow the problem down to a complexity of 2⁵⁶: “the security of MS-CHAPv2 can be reduced to the strength of a single DES encryption” [original emphasis].

Pico Computing’s key contribution to the effort is in the form of an FPGA-based box that can “crack any MS-CHAPv2 handshake in less than a day”, Marlinspike writes.

Marlinspike says that MS-CHAPv2 should be purged from the Internet, advising that PPTP traffic “should be considered unencrypted”, and that MS-CHAPv2 enterprise users should begin migrating – now. ®

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