Microsoft warns of 'man-in-the-middle' VPN password hack

'Not a security vulnerability that requires a security update,' says company

Microsoft yesterday warned Windows users of possible "man-in-the-middle" attacks able to steal passwords for some wireless networks and VPNs, or virtual private networks.

It won't issue a security update for the problem, however.

The security advisory was Microsoft's reaction to a disclosure several weeks ago by security researcher Moxie Marlinspike at the Defcon conference.

In a blog post written shortly after his Defcon talk, Marlinspike explained his interest in MS-CHAP v2 (Microsoft Challenge Handshake Authentication Protocol version 2). "Even as an aging protocol with some prevalent criticism, it's still used quite pervasively," Marlinspike said. "It shows up most notably in PPTP VPNs, and is also used quite heavily in WPA2 Enterprise environments."

At the same time, Marlinspike published "Chapcrack," a tool that parses data for passwords encrypted with MS-CHAP v2, then decodes them using the CloudCracker password cracking service.

Microsoft acknowledged the threat. "An attacker who successfully exploited these cryptographic weaknesses could obtain user credentials," the Monday advisory stated. "Those credentials could then be re-used to authenticate the attacker to network resources, and the attacker could take any action that the user could take on that network resource."

MS-CHAP v2 is used to authenticate users in PPTP-based (Point-to-Point Tunneling Protocol) VPNs. Windows includes a built-in implementation of PPTP.

To use Chapcrack, an attacker must first capture data packets being transmitted over a VPN or Wi-Fi. The most likely scenario: Spoofing a legitimate wireless hotspot, say at an airport, to sniff out VPN or other traffic, then grab it out of the air.

But Microsoft won't ship an update to fix the problem. "This is not a security vulnerability that requires Microsoft to issue a security update," Monday's advisory read. "This issue is due to known cryptographic weaknesses in the MS-CHAP v2 protocol and is addressed through implementing configuration changes."

Instead, Microsoft recommended that IT administrators add PEAP (Protected Extensible Authentication Protocol) to secure passwords for VPN sessions. A support document described how to configure servers and clients for PEAP.

As Marlinspike noted, MS-CHAP v2, which harks back to Windows NT SP4 and Windows 98, has been denounced as insecure for years, primarily because it's been vulnerable to "dictionary" attacks, where hackers try large numbers of possible passwords.

Windows 7 supports MS-CHAP v2, as does Windows XP and Vista, and Windows Server 2003, Server 2008 and Server 2008 R2.

MS-CHAP v2 cracking tools go back at least to 2007 with the publication of AsLEAP 2.1.

Microsoft said it had not seen any in-the-wild attacks using Marlinspike's Chapcrack.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed. His email address is

See more by Gregg Keizer on

Read more about cybercrime and hacking in Computerworld's Cybercrime and Hacking Topic Center.

Join the CSO newsletter!

Error: Please check your email address.
Show Comments

Featured Whitepapers

Editor's Recommendations

Solution Centres

Stories by Gregg Keizer

Latest Videos

  • 150x50

    CSO Webinar: Will your data protection strategy be enough when disaster strikes?

    Speakers: - Paul O’Connor, Engagement leader - Performance Audit Group, Victorian Auditor-General’s Office (VAGO) - Nigel Phair, Managing Director, Centre for Internet Safety - Joshua Stenhouse, Technical Evangelist, Zerto - Anthony Caruana, CSO MC & Moderator

    Play Video

  • 150x50

    CSO Webinar: The Human Factor - Your people are your biggest security weakness

    ​Speakers: David Lacey, Researcher and former CISO Royal Mail David Turner - Global Risk Management Expert Mark Guntrip - Group Manager, Email Protection, Proofpoint

    Play Video

  • 150x50

    CSO Webinar: Current ransomware defences are failing – but machine learning can drive a more proactive solution

    Speakers • Ty Miller, Director, Threat Intelligence • Mark Gregory, Leader, Network Engineering Research Group, RMIT • Jeff Lanza, Retired FBI Agent (USA) • Andy Solterbeck, VP Asia Pacific, Cylance • David Braue, CSO MC/Moderator What to expect: ​Hear from industry experts on the local and global ransomware threat landscape. Explore a new approach to dealing with ransomware using machine-learning techniques and by thinking about the problem in a fundamentally different way. Apply techniques for gathering insight into ransomware behaviour and find out what elements must go into a truly effective ransomware defence. Get a first-hand look at how ransomware actually works in practice, and how machine-learning techniques can pick up on its activities long before your employees do.

    Play Video

  • 150x50

    CSO Webinar: Get real about metadata to avoid a false sense of security

    Speakers: • Anthony Caruana – CSO MC and moderator • Ian Farquhar, Worldwide Virtual Security Team Lead, Gigamon • John Lindsay, Former CTO, iiNet • Skeeve Stevens, Futurist, Future Sumo • David Vaile - Vice chair of APF, Co-Convenor of the Cyberspace Law And Policy Community, UNSW Law Faculty This webinar covers: - A 101 on metadata - what it is and how to use it - Insight into a typical attack, what happens and what we would find when looking into the metadata - How to collect metadata, use this to detect attacks and get greater insight into how you can use this to protect your organisation - Learn how much raw data and metadata to retain and how long for - Get a reality check on how you're using your metadata and if this is enough to secure your organisation

    Play Video

  • 150x50

    CSO Webinar: How banking trojans work and how you can stop them

    CSO Webinar: How banking trojans work and how you can stop them Featuring: • John Baird, Director of Global Technology Production, Deutsche Bank • Samantha Macleod, GM Cyber Security, ME Bank • Sherrod DeGrippo, Director of Emerging Threats, Proofpoint (USA)

    Play Video

More videos

Blog Posts

Market Place