
   Protocol Security

   Summary

                                                         by Peter Mueller

   PPTP is known to be a faulty protocol. The designers of the protocol,
   Microsoft, recommend not to use it due to the inherent risks. Lots of
   people use PPTP anyway due to ease of use, but that doesn't mean it is
   any less hazardous. The maintainers of PPTP Client and Poptop
   recommend using OpenVPN (SSL based) or IPSec instead.

   (Posted on [1]2005-08-10 to the [2]mailing list)
     _________________________________________________________________

   Why not use PPTP?

                                                         by James Cameron

   The point to point tunneling protocol (PPTP) is not secure enough for
   some information security policies.

   It's the nature of the MSCHAP V2 authentication, how it can be broken
   trivially by capture of the datastream, and how MPPE depends on the
   MSCHAP tokens for cryptographic keys. MPPE is also only 128-bit,
   reasonably straightforward to attack, and the keys used at each end
   are the same, which lowers the effort required to succeed. The obvious
   lack of two-factor authentication, instead relying on a single
   username and password, is also a risk. The increasing use of domestic
   wireless systems makes information capture more likely.

   However, that doesn't mean people don't accept the risks. There are
   many corporations and individuals using PPTP with full knowledge of
   these risks. Some use mitigating controls, and some don't.

   Many people seem to judge the security of a protocol by the
   availability of the implementation, the ease of installation, or the
   level of documentation on our web site. Improving the documentation is
   the purpose of this web site, and we aren't doing that in order to say
   anything about the risks of the software! Any judgement of security
   should be rigorously applied to the design and implementation alone.

   PPTP on Linux, and Microsoft's PPTP, both implement fixes for
   vulnerabilities that were detected years ago in Microsoft's PPTP. But
   there remain the design vulnerabilities that cannot be fixed without
   changing the design. The changes needed would break interoperability.
   We can't change the Linux PPTP design, because it would stop working
   with Microsoft PPTP. They can't change their design, because it would
   stop working with all the other components out there, such as Nortel
   and Cisco, embedded routers, ADSL modems and their own Windows
   installed base.

   The only option then is to deprecate the product and promote the
   replacement. Microsoft promote something else. Our choice for Open
   Source systems is OpenVPN or IPsec.

   Level of acceptance isn't a good indicator of risk either. Some have
   said that the shipping of MSCHAP V2, MPPE and PPTP in Linux
   distributions is an indication of design security, but that's not the
   reason. It's for interoperability. As an example, see how Linux
   distributions still ship telnet, ftp, and rsh, even though these
   components are insecure because they reveal the password in cleartext
   in the network packets. The same can be said of many other components
   and packages.

   Our recommendations are;

    1. do not implement PPTP between open source systems, because there's
       no justification, better security can be had from OpenVPN or
       IPsec,

    2. do not implement PPTP servers unless the justification is that the
       clients must not have to install anything to get going (Microsoft
       PPTP is included already), and be aware of the risks of
       information interception,

    3. do not implement PPTP clients unless the justification is that the
       server only provides PPTP, and there's nothing better that can be
       used, and again be aware of the risks of information interception.

   (Posted on [3]2005-08-10 to the [2]mailing list)

References

   1. http://marc.theaimsgroup.com/?l=poptop-server&m=112369621702624&w=2
   2. http://pptpclient.sourceforge.net/contact.phtml#list
   3. http://marc.theaimsgroup.com/?l=poptop-server&m=112365342910897&w=2
