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Na implementação da Microsoft, o tráfego PPP tunelado pode ser autenticado utilizando [[Password authentication protocol|PAP]], [[/Point-to-Point Protocol#CHAP|CHAP]], ou [[MS-CHAP|MS-CHAP v1/v2]].
==Segurança==
O PPTP hasfoi beenalvo thede subjectdiversas ofanálises manyde securitysegurança analysese andsérias seriousvulnerabilidades securityforam vulnerabilitiesdescobertas haveno beenprotocolo. foundAs invulnerabilidades theconhecidas protocol.estão Therelacionadas knowncom vulnerabilitiesos relateprotocolos tode theautenticação underlyingutilizados PPPem authenticationconjunto protocolscom usedo PPP, theo designprojeto ofdo theprotocolo MPPE protocolassim ascomo wella asintegração theentre integrationPPP betweene MPPE anddurante PPPa authenticationautenticação fore sessionestabelecimento keyde establishment.chaves de sessão<ref>{{ citecitar web| titletitulo=Microsoft saysdiz don't"não use o PPTP ande MS- CHAPCHAM"(em inglês)|url=http://www.h-online.com/security/news/item/Microsoft-says-don-t-use-PPTP-and-MS-CHAP-1672257.html| accessdateacessadoem=2012-11-03}}</ref><ref>{{ citecitar web| titletitulo= AGolpe deathmortal blowno forPPTP(em PPTPinglês)|url=http://www.h-online.com/security/features/A-death-blow-for-PPTP-1716768.html| accessdateacessadoem=2012-11-03}}</ref><ref>{{ citecitar newsweb| titletitulo= DifferencesDiferenças betweenentre PPTP ande L2TP (em inglês)|url=https://www.bestvpnrating.com/blog/main-differences-between-pptp-l2tp| accessdateacessadoem= 7 August 2016 |work=bestvpnrating-08-07}}</ref> .▼
Um sumário destas vulnerabilidades encontra-se abaixo:
* O [[MS-CHAP]]-v1 isé fundamentallyfundalmentalmente insecureinseguro. ToolsFerramentas existexistem topara triviallyde extractforma thetrivial extrair hashes de senhas do NT Passwordapóx hashesuma fromanálise ade capturedtráfego MSCHAP-v1 exchangecapturado.<ref name="schneier1">[http://www.schneier.com/paper-pptp.pdf Bruce Schneier, '' CryptanalysisCriptoanálise ofdo Microsoft'sPPTP Pointda to Point Tunneling Protocol (PPTP)Microsoft''].</ref> ▼* Ao utilizar o MS-CHAP-v1, o [[Microsoft Point-to-Point Encryption|MPPE]] utiliza a mesma chave de sessão RC4 para encriptar tráfego nos dois sentidos do fluxo. Este tráfego pode ser criptoanalisado com métodos padrão efetuando [[XOR]] dos fluxos de dados em ambas as direções para extrair informação<ref>[http://www.schneier.com/paper-pptpv2.pdf Bruce Schneier, ''Criptoanálise do PPTP da Microsoft e Extensões de Autenticação(MS-CHAPv2)'', 19 de Outubro de 1999].</ref>
* MS-CHAP-v2 é vulnerável a [[ataque de dicionário|ataques de dicionário]] em pacotes de resposta à desafio capturados. Ferramentas existem para efetuar este processo rapidamente. <ref>Josh do willhackforsushi.com "[http://www.willhackforsushi.com/Asleap.html notícias]".</ref>
* InEm 2012, itfoi wasdemonstrado demonstratedque thata thecomplexidade complexityde ofum aataque brute-forcede attackforça onbruta aem uma chave MS-CHAP-v2 keyé isequivalente equivalenta toum aataque brute-forcede attackmesmo ontipo aem singleuma única chave [[Data Encryption Standard|DES]] key. AnUm serviço online servicedemonstrou wasque alsoé demonstratedcapaz whichde isde capabledecriptar ofuma decrypting asenha MS-CHAP-v2 MD4 passphrase inem 23 hourshoras.<ref>{{ citecitar web|url=https://www.cloudcracker.com/blog/2012/07/29/cracking-ms-chap-v2/ | titletitulo= DivideDividir andpara ConquerConquistar: CrackingQuebrando o MS-CHAPv2 with acom 100% successde rateacerto(em inglês)| publisherobra=Cloudcracker.com | datedata=2012-07-29 | accessdateacessadoem=2012-09-07}}</ref><ref>{{ citecitar web|url=http://www.theregister.co.uk/2012/07/31/ms_chapv2_crack/ | titletitulo=Marlinspike demosdemonstra quebra do MS-CHAPv2 (em crack inglês)| publisherobra=[[The Register]] | datedata=2012-07-31 | accessdateacessadoem=2012-09-07}}</ref> .▼* O MPPE utiliza o [[RC4]] como cifra para o fluxo encriptado. Não existe um método para a autenticação do fluxo de texto cifrado portanto, este é vulnerável a ataques do tipo bit-flipping. Um atacante pode modificar o fluxo em trânsito e ajustar bits únicos para mudar o conteúdo de saída do fluxo sem que seja detectado. Estas trocas de bit podem ser detectados por outros protocolos através de checksums ou outros métodos.<ref name="schneier1"/>
[[Extensible Authentication Protocol|EAP-TLS]] é visto como uma alternativa superior para a autenticação do PPTP;<ref>[http://technet.microsoft.com/en-us/library/cc739638%28WS.10%29.aspx Escolhendo [[EAP-TLS]] ou MS-CHAP v2 para autenticação de usuário], Microsoft TechNet, 28 de Março de 2003</ref> contudo, requer a implementação de [[PKI]] com certificados no cliente e no servidor. Este fato pode tornar inviável esta forma de autenticação para algumas localidades em que a instalação remota é necessária.
{{referências}}
[[Categoria:Protocolos de tunelamento]]
[[Categoria:Redes de computadores]]
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== Protocol description ==
A PPTP tunnel is instantiated by communication to the peer on [[Transport Control Protocol|TCP]] port 1723. This TCP connection is then used to initiate and manage a second [[Generic Routing Encapsulation|GRE]] tunnel to the same peer.
The PPTP GRE packet format is non standard, including an additional ''acknowledgement'' 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 [[Internet Protocol|IP]], [[NetBEUI]] and [[IPX]].
In the Microsoft implementation, the tunneled PPP traffic can be authenticated with [[Password authentication protocol|PAP]], [[Challenge-handshake authentication protocol|CHAP]], [[MS-CHAP|MS-CHAP v1/v2]] .
==Security==
▲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.<ref>{{cite web|title=Microsoft says don't use PPTP and MS-CHAP|url=http://www.h-online.com/security/news/item/Microsoft-says-don-t-use-PPTP-and-MS-CHAP-1672257.html|accessdate=2012-11-03}}</ref><ref>{{cite web|title=A death blow for PPTP|url=http://www.h-online.com/security/features/A-death-blow-for-PPTP-1716768.html|accessdate=2012-11-03}}</ref><ref>{{cite news|title=Differences between PPTP and L2TP|url=https://www.bestvpnrating.com/blog/main-differences-between-pptp-l2tp|accessdate=7 August 2016|work=bestvpnrating}}</ref>
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.<ref name="schneier1">[http://www.schneier.com/paper-pptp.pdf Bruce Schneier, ''Cryptanalysis of Microsoft's Point to Point Tunneling Protocol (PPTP)''].</ref>
* When using MS-CHAP-v1, [[Microsoft Point-to-Point Encryption|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.<ref>[http://www.schneier.com/paper-pptpv2.pdf Bruce Schneier, ''Cryptanalysis of Microsoft's PPTP Authentication Extensions (MS-CHAPv2)'', October 19 1999].</ref>
* MS-CHAP-v2 is vulnerable to dictionary attacks on the captured challenge response packets. Tools exist to perform this process rapidly.<ref>Josh of willhackforsushi.com "[http://www.willhackforsushi.com/Asleap.html news]".</ref>
▲* 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 [[Data Encryption Standard|DES]] key. An online service was also demonstrated which is capable of decrypting a MS-CHAP-v2 MD4 passphrase in 23 hours.<ref>{{cite web|url=https://www.cloudcracker.com/blog/2012/07/29/cracking-ms-chap-v2/ |title=Divide and Conquer: Cracking MS-CHAPv2 with a 100% success rate |publisher=Cloudcracker.com |date=2012-07-29 |accessdate=2012-09-07}}</ref><ref>{{cite web|url=http://www.theregister.co.uk/2012/07/31/ms_chapv2_crack/ |title=Marlinspike demos MS-CHAPv2 crack |publisher=[[The Register]] |date=2012-07-31 |accessdate=2012-09-07}}</ref>
* 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.<ref name="schneier1"/>
[[Extensible Authentication Protocol|EAP-TLS]] is seen as the superior authentication choice for PPTP;<ref>[http://technet.microsoft.com/en-us/library/cc739638%28WS.10%29.aspx Choosing [[EAP-TLS]] or MS-CHAP v2 for User-Level Authentication], Microsoft TechNet, March 28, 2003</ref> 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.
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