Point-to-Point Tunneling Protocol
The PPTP specification does not describe encryption or authentication features and relies on the Point-to-Point Protocol being tunneled to implement security functionality. However the most common PPTP implementation, shipping with the Microsoft Windows product families, implements various levels of authentication and encryption natively as standard features of the Windows PPTP stack. The intended use of this protocol is to provide security levels and remote access levels comparable with typical VPN products.
|Internet protocol suite|
A specification for PPTP was published in July 1999 as RFC 2637 and was developed by a vendor consortium formed by Microsoft, Ascend Communications (today part of Alcatel-Lucent), 3Com, and others. PPTP has not been proposed nor ratified as a standard by the IETF.
The PPTP GRE packet format is non standard, including an additional acknowledgement field replacing the typical routing field in the GRE header. However, like in a normal GRE connection, those modified GRE packets are directly encapsulated into IP packets, and seen as IP protocol number 47.
In the Microsoft implementation, the tunneled PPP traffic can be authenticated with PAP, CHAP, MS-CHAP v1/v2 or EAP-TLS. The PPP payload is encrypted using Microsoft Point-to-Point Encryption (MPPE) when using MS-CHAPv1/v2 or EAP-TLS. MPPE is described by RFC 3078.
PPTP was the first VPN protocol that was supported by Microsoft Dial-up Networking. All releases of Microsoft Windows since Windows 95 OSR2 are bundled with a PPTP client, although they are limited to only 2 concurrent outbound connections. Microsoft Windows Mobile 2003 and higher also support the PPTP protocol. The Routing and Remote Access Service for Microsoft Windows contains a PPTP server. The Microsoft implementation uses single DES in the MS-CHAP authentication protocol which many find unsuitable for data protection needs.
Windows Vista and later support the use of PEAP with PPTP. The authentication mechanisms supported are PEAPv0/EAP-MSCHAPv2 (passwords) and PEAP-TLS (smartcards and certificates). Windows Vista removed support for using the MSCHAP-v1 protocol to authenticate remote access connections.
Linux server-side support for PPTP is provided by the PoPToP daemon and kernel modules for PPP and MPPE. The first PPTP implementation was developed by Matthew Ramsay in 1999 and initially distributed under the GNU GPL by Moreton Bay. However, Linux distributions initially lacked full PPTP support because MPPE was believed to be patent encumbered. Full MPPE support was added to the Linux kernel in the 2.6.14 release on October 28, 2005. SuSE Linux 10 was the first Linux distribution to provide a complete working PPTP client. There is also ACCEL-PPP – PPTP/L2TP/PPPoE server for Linux which supports PPTP in kernel-mode.
OS X and iOS are bundled with a PPTP client. Cisco and Efficient Networks sell PPTP clients for older Mac OS releases. Palm PDA devices with Wi-Fi are bundled with the Mergic PPTP client.
PPTP has been the subject of many security analyses and serious security vulnerabilities have been found in the protocol. The known vulnerabilities relate to the underlying PPP authentication protocols used, the design of the MPPE protocol as well as the integration between MPPE and PPP authentication for session key establishment. PPTP is (as of Oct 2012) considered cryptographically broken and its use is no longer recommended by Microsoft.
A summary of these vulnerabilities is below:
- MSCHAP-v1 is fundamentally insecure. Tools exist to trivially extract the NT Password hashes from a captured MSCHAP-v1 exchange.
- When using MSCHAP-v1, MPPE uses the same RC4 session key for encryption in both directions of the communication flow. This can be cryptanalysed with standard methods by XORing the streams from each direction together.
- MSCHAP-v2 is vulnerable to dictionary attack on the captured challenge response packets. Tools exist to perform this process rapidly.
- In 2012 it was shown that brute-force attack on MSCHAP-v2 is equivalent to single DES key brute-force attack. Online service was presented, which is capable to restore MSCHAP-v2 passphrase's MD4 in 23 hours.
- MPPE uses RC4 stream cipher for encryption. There is no method for authentication of the ciphertext stream and therefore the ciphertext is vulnerable to a bit-flipping attack. An attacker could modify the stream in transit and adjust single bits to change the output stream without possibility of detection. These bit flips may be detected by the protocols themselves through checksums or other means.
EAP-TLS is seen as the superior authentication choice for PPTP; however, it requires implementation of a Public Key Infrastructure for both client and server certificates. As such it is not a viable authentication option for many remote access installations.
See also 
- RFC 2637
- Ray, Marsh (April 3, 2012). "MS PPTP MPPE only as secure as *single* DES". Retrieved April 3, 2012.
- "The MS-CHAP version 1 authentication protocol has been deprecated in Windows Vista". Support.microsoft.com. 2007-03-15. Retrieved 2012-02-07.
- "PoPPToP Linux daemon at SourceForge". Sourceforge.net. Retrieved 2012-02-07.
- "Linux Journal PPTP Implementation". Linuxjournal.com. Retrieved 2012-02-07.
- "ACCEL-PPP – PPTP/L2TP/PPPoE server for Linux". Accel-ppp.sourceforge.net. Retrieved 2012-02-07.
- "Microsoft says don't use PPTP and MS-CHAP". Retrieved 2012-11-03.
- "A death blow for PPTP". Retrieved 2012-11-03.
- Bruce Schneier, Cryptanalysis of Microsoft's Point to Point Tunneling Protocol (PPTP).
- Bruce Schneier, Cryptanalysis of Microsoft's PPTP Authentication Extensions (MS-CHAPv2), October 19 1999.
- Josh of willhackforsushi.com "news".
- "Divide and Conquer: Cracking MS-CHAPv2 with a 100% success rate". Cloudcracker.com. 2012-07-29. Retrieved 2012-09-07.
- "Marlinspike demos MS-CHAPv2 crack". The Register. 2012-07-31. Retrieved 2012-09-07.
- Choosing EAP-TLS or MS-CHAP v2 for User-Level Authentication, Microsoft TechNet, March 28, 2003
- Windows NT: Understanding PPTP from Microsoft
- FAQ on security flaws in Microsoft's implementation, Bruce Schneier, 1998
- Cryptanalysis of Microsoft's PPTP Authentication Extensions (MS-CHAPv2), Bruce Schneier, 1999