Microsoft pptp vpn vulnerabilities Exploits in Action

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Key fingerprint = AF19 FA27 2F94 998D FDB5 DE3D F8B5 06E4 A169 4E46 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Key fingerprint = AF19 FA27 2F94 998D FDB5 DE3D F8B5 06E4 A169 4E46 
 
© SANS Institute 2000 - 200
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Author retains full rights.
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compares the server's AR to the client's AR, if they match, then server is 
authenticated to client 
MPPE keys are now based on the MS-CHAP v2 information with a 
•
unique key for the server and a unique one for the client (compared to the 
same key for each in v1)
See the Counterpane Labs document “Cryptanalysis of Microsoft's MS 
CHAP v2” for even more detailed information on these steps.
Unfortunately the following well published weakness were not addressed:
MS-CHAP NT hash is still easily vulnerable to cracking common 
•
passwords using basic dictionary attacks
MPPE still does not provide true 40 bit or 128 bit encryption
•
MPPE still does not encrypt the NCP PPP packets
•
MPPE is still vulnerable to bit-flipping attacks
•
And by default (requires editing the registry to prevent this attack) the 
•
client and server can be susceptible to version rollback attacks to make 
them use MS-CHAP v1 instead of v2, making the LANMAN hash 
available to the attacker once again
Typical PPTP traffic captured using ethereal on Linux:
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