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What is PPTP?

What is PPTP? Is it a good VPN solution?

The PPTP specification was originally developed by a consortium that included Ascend Communications, 3Com/Primary Access, ECI Telematics, U.S. Robotics and Microsoft. The protocol was originally designed as an encapsulation mechanism, to allow the transport of non-TCP/IP protocols (such as IPX) over the Internet using Generic Routing Encapsulation (GRE). The specification itself is fairly generic, and allows for a variety of authentication mechanisms and encryption algorithms. Note that these security features were added later, not built in from the beginning.

Several vendors have created PPTP systems. However, the vast majority of PPTP users implement the Microsoft version. The following discussion of PPTP security issues are specific to the Microsoft implementation, which features:

  • PPTP server -- NT 4.0 or later
  • PPTP clients -- Win 95/98/NT; WFW, Macintosh with 3rd party hardware
  • Authentication/authorization mechanisms limited to NT domain security; manage access to non-NT domain resources via network segregation, RADIUS (maybe)

    PPTP can be used to control access to the private network via NT domain security controls (user- and group-level access to domain resources), and by segregating resources on the corporate network. With the release of the Internet Authentication Services update for NT 4.0, RADIUS may be used to perform PPTP authentication -- but it is unknown whether or not the authorization and access control features of RADIUS are also supported.

    Use of PPTP requires that IP forwarding be enabled on the NT server.

    Setting up a PPTP system requires configuring the Remote Access Server capability on the NT server, adding routing functionality to the RAS system, applying several newly-released security patches, and configuring the PPTP-specific registry keys. And hardening the server itself.

    Security Concerns:
  • Flawed encryption mechanism -- non-random keys, session keys weak hash of user password, key lengths too short (non-configurable)
  • Bad password management in mixed Win95/NT environment; static passwords easily compromised
  • Vulnerable to server spoofing attacks because packet authentication not implemented, easy denial-of-service attacks even inside firewalls
  • MS claims cryptographic weaknesses not yet exploited

    The initial release of PPTP used the MSCHAP mechanism for end-user authentication. After numerous criticisms that MSCHAP was easily compromised, especially in situations when Windows 95 was the client operating system, Microsoft released a patch to the original authentication protocol. To quote the Microsoft WebSite: "This new protocol provides mutual authentication, stronger initial data encryption keys, and different encryption keys for the transmit and receive paths. To minimize the risk of password compromise during MSCHAP exchanges, MSCHAP V2 drops support for the MSCHAP password change V1, and will not transmit the LMHash encoding of the password. ...For VPN connection requests, a Windows NT server will offer MSCHAP V2 before offering the legacy MSCHAP. Updated Windows clients (all platforms) will accept MSCHAP V2 when it is offered." (August 18, 1998) Microsoft also added a new registry key, SecureVPN, that forces incoming VPN connection requests to use the new authentication mechanism. These changes should prevent a PPTP client from indicating using the older, LMHash mechanism. However, the effectiveness of these patches has not yet been verified by any independent reviewer.

    Also note that although Microsoft describes PPTP as using either 40-bit or 128-bit encryption, their use of the user's password to create a session key, rather than a randomly generated key, greatly reduces the strength of the encryption process. None of the recent security releases addresses this issue.

    Microsoft claims to have improved the mechanism that generates session keys (which is based on a hash of the user's password). If this is true, it helps protect against hijacking attacks, as well as making brute force crypto attacks harder. NB: even this enhancement does not improve the cryptographic weakness, which is based on the flawed decision to use passwords to generate keys. Remember, no matter how strong an encryption algorithm is, it can be compromised via a brute-force attack. The only protection against brute force is a long key length, with purely random keys - not what Microsoft has implemented. And again, this enhancement has not been verified (as of November 1998) by any third-party evaluator.

    And of course, there are potential issues with getting GRE through a lot of commercial firewalls, and lots of problems with technical support on a system that could rapidly become mission-critical.

    So no, the VPN list moderator doesn't think that PPTP is a reasonable VPN solution, at least from the security point of view. Your mileage may vary.

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