To get the XAUTH based authentication information (that is the part where
the RADIUS Server is involved) you must start a man in the middle attack
and this MITM attack is only possible when you've already cracked the
preconfigured preshared key and when you are in physical position to
perform a MITM attack (that's really not too easy).

Hope that helps ;-

At 21:10 23.04.03 -0400, Gary Flynn wrote:
>Damir Rajnovic wrote:
>
>>Please note that the same class of attack is possible even if Xauth
>>(Extended Authentication) is used. This is because Xauth is performed
>>after Phase 1 is completed and, for this attack, an adversary
>>needs only a packet from Phase 1. Furthermore, after the pre-shared key
>>has been discovered, an adversary can mount an active MiTM attack
>>on Xauth. The outcome depends on the exact authentication method used
>>in Xauth.
>May I ask how this applies to "IKE Shared Secret AAA"?
>If a Radius backend authenticator is used, is the shared key
>that is vulnerable one preconfigured in the VPN hosts or one
>based on the Radius password? The reason I ask is that the
>preconfigured shared key can easily be lengthened but the
>backend password, if based on end user passwords, is not
>as easy a solution. :)
>
>Thank you.
>
>Gary Flynn
>Security Engineer - James Madison University
>

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