[SR-Users] Call-ID Uniqueness and safety?

[Daniel-Constantin Mierla]

Hello,

there are many things that can go wrong if a bad actor can forge sip
messages. That's why the call has to be authenticated/authorized in the
first place (either user/password or some other trusted relationship by
IP, certificate, ...).

Now if a trusted actor tries to do bad things, then it is harder to
prevent, but once discovered, the identity is known and it can be
eventually published in a way or another (by laws and the commercial
agreement).

Normally, the call-id should be randomly generated, with very low
chances of overlapping during the same time frame of active call. If
rtpengine is adding its own random part to file names, then the risk of
overwriting later is also very low.

The dialog is identified by call-id, from tag (set by caller) and to tag
(set by callee), so in this case both caller and callee have to be bad
actors.

But in many cases call-id is used alone as a key. Adding the from tag is
quite useless, because is set by the same UA, so if it wants to send
same call-id, can do the same with from tag. Then the to-tag is set by
the called side and during the call initiations, there can be many
outgoing branches, so during the 1xx phase there can be a couple of
to-tags flowing around. So if something needs to be stored in the early
phase, the to-tag of the to-be-answered call is not yet know.

Rejecting new calls having the same callid as another recent one could
be safety check to plug if you are really concerned and you don't trust
completely who is making the call. You can easily use a htable for that,
to cache used call-ids and block new calls with same value.

Cheers,
Daniel

On 13.02.20 10:07, Mark Boyce wrote:
> Hi all
>
> Over coffee we’ve been discussing the uniqueness and safety of call-id this morning.  Lots of things like RTPEngine recording use call-id to generate filenames (yes it does escape them first :-). Not to mention dialog tracking (also uses from/to tags) etc.
>
> Which got us thinking.  As call-id is determined by the first SIP packet that normally comes from a UA, should we care if this UA was broken or worse malicious?  
>
> What would happen if they were repeated?  For example we may struggle to determine a recordings owner in RTPEngine (file name format is callid+random.wav).
>
> For security should we be rejecting requests where we’ve seen the call-id before?
>
>
> All theoretical over coffee, as it was that or start working!
>
> Just wondering what peoples thoughts were?
>
> Cheers
> Mark
> _______________________________________________
> Kamailio (SER) - Users Mailing List
> sr-users at lists.kamailio.org
> https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users

-- 
Daniel-Constantin Mierla -- www.asipto.com
www.twitter.com/miconda -- www.linkedin.com/in/miconda
Kamailio Advanced Training - March 9-11, 2020, Berlin - www.asipto.com
Kamailio World Conference - April 27-29, 2020, in Berlin -- www.kamailioworld.com