[OpenSIPS-Users] Accounting: How to avoid a fraudulent BYE with lower CSeq?

[Dan Pascu]

There is more than one way to skin a cat as they say and this case is no 
exception. There are multiple ways to send a BYE that will not be 
accepted but still make the proxy end the session (at least in the 
current implementation):

1. Lower CSEQ number
2. Low max forwards value, so the BYE doesn't reach the gateway
3. Invalid destination, so it gives timeout after a while
4. Wrong call-id, from-tag, to-tag which will make the BYE be rejected by 
the gateway

All these have in common that they will get a negative reply to the BYE. 
So the idea would be not to not the dialog unless a 200 OK for the BYE is 
received.

This can be worked around however too. I can send the BYE with all the 
dialog identification elements and the correct CSEQ number, but to a 
different ruri, pointing back to myself, and I can reply with 200 OK to 
the BYE. To avoid this the proxy would have to check the ruri as well.

But then I can send one with the proper ruri, but a different route set 
that puts me in the front of the gateway, so when I receive the BYE, 
instead of forwarding it to the gateway as the route set requests, I 
reply myself with a 200 OK making it look like it came from the gateway.

In the end it means, the proxy will have to verify everything (dialog 
identification elements, cseq, ruri, route set) to avoid fraud and also 
wait for a 200 OK, which makes it look more like a b2bua after all

In the end the simplest solution is to eliminate the incentive for users 
to fraud that system, not to put bigger or more locks on it. This can be 
done by using a flat-fee model, but I agree that lobbying for this will 
fell on deaf ears with the big players that own the gateways. Still this 
is the simplest solution that not only eliminates the incentive for 
fraud, but also simplifies the over complex billing system and eliminates 
the burden the billing puts on a system with millions of users that have 
to be accounted in realtime.

On Wednesday 07 January 2009, Adrian Georgescu wrote:
> The dialog module could eventually be used to detect out of sync Cseq
> and take decision to terminate the call. Is this feasible?
>
> Adrian
>
> On Dec 19, 2008, at 3:59 PM, Victor Pascual Ávila wrote:
> > On Fri, Dec 19, 2008 at 3:22 PM, Bogdan-Andrei Iancu
> >
> > <bogdan at voice-system.ro> wrote:
> >> Hi Iñaki,
> >>
> >> Have you consider requesting auth for the BYE ? from SIP point of
> >> view
> >> is perfectly valid....
> >
> > I'm afraid this would only prevent external attackers but does not
> > protect you from your own customers-- guys who have the credentials
> > and wanna call for free.
> >
> > Cheers,
> > --
> > Victor Pascual Ávila
> > _______________________________________________
> > Users mailing list
> > Users at lists.opensips.org
> > http://lists.opensips.org/cgi-bin/mailman/listinfo/users



-- 
Dan