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PPTP is the only VPN protocol supported by some devices (for example, the Asus RT-AC66U WiFi router). If PPTP is configured to only use the most secure options, does its use present any security vulnerabilities?

The most secure configuration of PPTP is to exclusively use:

  • MPPE-128 encryption (which uses RC4 encryption with a 128bit key)
  • MS-CHAPv2 authentication (which uses SHA-1)
  • strong passwords (minimum 128 bits of entropy)

I realize that RC4 and SHA-1 have weaknesses, but I am interested in practical impact. Are there known attacks or exploits that would succeed against a PPTP VPN with the above configuration?

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1 Answer 1

up vote 7 down vote accepted

Yes. The protocol itself is no longer secure, as cracking the initial MS-CHAPv2 authentication can be reduced to the difficulty of cracking a single DES 56-bit key, which with current computers can be brute-forced in a very short time.

The attacker can do a MITM to capture the handshake (and any PPTP traffic after that), do an offline crack of the handshake and derive the RC4 key. Then, the attacker will be able to decrypt and analyse the traffic carried in the PPTP VPN.

Additionally, PPTP provides weak protection to the integrity of the data being tunneled. The RC4 cipher, while providing encryption, does not verify the integrity of the data as it is not an Authenticated Encryption with Associated Data (AEAD) cipher. PPTP is hence vulnerable to bit-flipping attacks, ie. the attacker can modify PPTP packets without possibility of detection.

For more information, see http://www.computerworld.com/s/article/9229757/Tools_released_at_Defcon_can_crack_widely_used_PPTP_encryption_in_under_a_day and How can I tell if a PPTP tunnel is secure?.

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