The Point-to-Point Tunneling Protocol (PPTP) is an obsolete method for implementing virtual private networks. PPTP has many well known security issues.
PPTP uses a TCP control channel and a Generic Routing Encapsulation tunnel to encapsulate PPP packets. Many modern VPNs use various forms of UDP for this same functionality.
The PPTP specification does not describe encryption or authentication features and relies on the Point-to-Point Protocol being tunneled to implement any and all security functionalities.
The PPTP implementation that ships 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.
A specification for PPTP was published in July 1999 as RFC 2637[1] and was developed by a vendor consortium formed by Microsoft, Ascend Communications (today part of Nokia), 3Com, and others.
A PPTP tunnel is instantiated by communication to the peer on TCP port 1723. This TCP connection is then used to initiate and manage a GRE tunnel to the same peer. The PPTP GRE packet format is non standard, including a new acknowledgement number field replacing the typical routing field in the GRE header. However, as in a normal GRE connection, those modified GRE packets are directly encapsulated into IP packets, and seen as IP protocol number 47. The GRE tunnel is used to carry encapsulated PPP packets, allowing the tunnelling of any protocols that can be carried within PPP, including IP, NetBEUI and IPX.
In the Microsoft implementation, the tunneled PPP traffic can be authenticated with PAP, CHAP, MS-CHAP v1/v2 .
Securityedit
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.[2][3][4][5]
A summary of these vulnerabilities is below:
MS-CHAP-v1 is fundamentally insecure. Tools exist to trivially extract the NT Password hashes from a captured MSCHAP-v1 exchange.[6]
When using MS-CHAP-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.[7]
MS-CHAP-v2 is vulnerable to dictionary attacks on the captured challenge response packets. Tools exist to perform this process rapidly.[8]
In 2012, it was demonstrated that the complexity of a brute-force attack on a MS-CHAP-v2 key is equivalent to a brute-force attack on a single DES key. An online service was also demonstrated which is capable of decrypting a MS-CHAP-v2 MD4 passphrase in 23 hours.[9][10]
MPPE uses the 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.[6]
EAP-TLS is seen as the superior authentication choice for PPTP;[11] however, it requires implementation of a public-key infrastructure for both client and server certificates. As such, it may not be a viable authentication option for some remote access installations. Most networks that use PPTP have to apply additional security measures or be deemed completely inappropriate for the modern internet environment. At the same time, doing so means negating the aforementioned benefits of the protocol to some point.[12]
^. Cloudcracker.com. 2012-07-29. Archived from the original on 2016-03-16. 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
^"VPN Protocol Comparison: IKEv2 vs IKEv1 vs OpenVPN vs L2TP vs PPTP". VPN Unlimited Blog. 2018-05-14. Retrieved 2018-06-19.
External linksedit
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
December 15, 2023
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The Point to Point Tunneling Protocol PPTP is an obsolete method for implementing virtual private networks PPTP has many well known security issues PPTP uses a TCP control channel and a Generic Routing Encapsulation tunnel to encapsulate PPP packets Many modern VPNs use various forms of UDP for this same functionality The PPTP specification does not describe encryption or authentication features and relies on the Point to Point Protocol being tunneled to implement any and all security functionalities The PPTP implementation that ships 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 Contents 1 History 2 Description 3 Security 4 See also 5 References 6 External linksHistory editA specification for PPTP was published in July 1999 as RFC 2637 1 and was developed by a vendor consortium formed by Microsoft Ascend Communications today part of Nokia 3Com and others PPTP has not been proposed nor ratified as a standard by the Internet Engineering Task Force Description editA PPTP tunnel is instantiated by communication to the peer on TCP port 1723 This TCP connection is then used to initiate and manage a GRE tunnel to the same peer The PPTP GRE packet format is non standard including a new acknowledgement number field replacing the typical routing field in the GRE header However as in a normal GRE connection those modified GRE packets are directly encapsulated into IP packets and seen as IP protocol number 47 The GRE tunnel is used to carry encapsulated PPP packets allowing the tunnelling of any protocols that can be carried within PPP including IP NetBEUI and IPX In the Microsoft implementation the tunneled PPP traffic can be authenticated with PAP CHAP MS CHAP v1 v2 Security editPPTP 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 2 3 4 5 A summary of these vulnerabilities is below MS CHAP v1 is fundamentally insecure Tools exist to trivially extract the NT Password hashes from a captured MSCHAP v1 exchange 6 When using MS CHAP 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 7 MS CHAP v2 is vulnerable to dictionary attacks on the captured challenge response packets Tools exist to perform this process rapidly 8 In 2012 it was demonstrated that the complexity of a brute force attack on a MS CHAP v2 key is equivalent to a brute force attack on a single DES key An online service was also demonstrated which is capable of decrypting a MS CHAP v2 MD4 passphrase in 23 hours 9 10 MPPE uses the 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 6 EAP TLS is seen as the superior authentication choice for PPTP 11 however it requires implementation of a public key infrastructure for both client and server certificates As such it may not be a viable authentication option for some remote access installations Most networks that use PPTP have to apply additional security measures or be deemed completely inappropriate for the modern internet environment At the same time doing so means negating the aforementioned benefits of the protocol to some point 12 See also editIPsec Layer 2 Tunneling Protocol L2TP Secure Socket Tunneling Protocol SSTP OpenVPN open source software application that implements VPN WireGuard a simple and effective VPN implementationReferences edit RFC 2637 Malware FAQ Microsoft PPTP VPN Retrieved 2017 06 29 Microsoft says don t use PPTP and MS CHAP Retrieved 2012 11 03 A death blow for PPTP Retrieved 2012 11 03 Differences between PPTP and L2TP bestvpnrating Archived from the original on 14 September 2016 Retrieved 7 August 2016 a b Bruce Schneier Cryptanalysis of Microsoft s Point to Point Tunneling Protocol PPTP Archived 2011 06 04 at the Wayback Machine Bruce Schneier Cryptanalysis of Microsoft s PPTP Authentication Extensions MS CHAPv2 October 19 1999 Wright Joshua Asleap Retrieved 2017 11 01 Divide and Conquer Cracking MS CHAPv2 with a 100 success rate Cloudcracker com 2012 07 29 Archived from the original on 2016 03 16 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 VPN Protocol Comparison IKEv2 vs IKEv1 vs OpenVPN vs L2TP vs PPTP VPN Unlimited Blog 2018 05 14 Retrieved 2018 06 19 External links editWindows 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 Retrieved from https en wikipedia org w index php title Point to Point Tunneling Protocol amp oldid 1180505161, wikipedia, wiki, book, books, library,
