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PPTP Client

Protocol Security


by James Cameron


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 2005-08-10 to the 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 2005-08-10 to the mailing list)


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