Re: draft-state-sip-relay-attack-00

Am 06.03.2009 20:41 Uhr, schrieb Dale Worley:
> On Thu, 2009-03-05 at 14:40 +0100, Nils Ohlmeier wrote:
>> One thing which is not that obvious but is implictly a requirement for the
>> attack: the proxies has to challenge in-dialog requests. I do not see a
>> big benefit in challeging in-dialog requests as these are hopefully
>> rejected by the remote side if no matching dialog exists. If the UA would
>> know that his proxy does not challenge in-dialog requests it could simply
>> ignore the challenge :-)
>
> Except that there are legitimate uses for challenging in-dialog
> requests:  sipX uses it to allow a phone to transfer a caller to any
> destination that the executing phone has permission to call.  The first
> step of this process is that when the executing phone sends a REFER, the
> proxy challenges the REFER so that the executing phone attaches its
> credentials to the REFER.  The proxy then analyzes these credentials to
> determine the user that is responsible for the transfer operation, etc.
> Without the in-dialog challenge, there is no way for the proxy to
> determine the user that is responsible for transfer operation.

Good point. Except that a REFER is not relevant for this replay attack. 
So would re-phrase it: I do not see a big benefit in challeging 
in-dialog INVITE requests. :-)

Greetings
   Nils
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