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PROTOCOL-SECURITY
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PROTOCOL-SECURITY
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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