[VoiceOps] Mitigating SIP threats with SBC policies, configuration settings

[Ryan Delgrosso]

Hi Robert,
Nope no good reason not to ACL that interface. If its setup according to 
Oracle BCP then you would have the sip-interface set to agents-only and 
have an agent defined for the other end of the trunk and then an ACL 
entry setup for that agent. This would result in traffic from anywhere 
but that agent just being dropped. This would have no impact on 
distributed media from the peer, so no need to build an ACL entry or 
account for media sending endpoints, that will be handled dynamically by 
the SBC.

On the second issue, properly ACL'ing the trunking interface will 
mitigate these issues however if there are other interfaces that DO need 
to listen publicly to anything then its best to set the sip interface on 
those to only allow registered endpoints and then tune your demotion 
policies to properly punt offenders to a blacklist. You are correct 
about this sort of thing being a gateway to CPU issues, and proper use 
of the deny list function is key to mitigating that exposure. The deny 
list is implemented in the NIU so stopping offenders there will preserve 
CPU cycles.

Feel free to let me know if you need anymore clarity on this.

-Ryan

On 9/3/2014 8:49 AM, Robert Nystrom wrote:
> Hello Everyone,
>
> New to the list, so please take it easy on me :-)  I'm reviewing the 
> security configuration for a customer that is using ACME SBC for SIP 
> trunk to their carrier, and have some questions.  I thought you guys 
> on the list would have a lot of experience with ACME security 
> architecture and best practices recommendations.
>
> First:  Customer's ACME is visible from public Internet on udp/5060, 
> and SIP trunk is only being used to interconnect to SIP trunk carrier 
> for inbound and outbound dialing.  I've tested the SBC from the 
> Internet and it actually responds to INVITE and REGISTER messages 
> (with 403 Forbidden).  They are alsonot supposed to be allowing any 
> REGISTER for remote user MD5 Digest Authentication - but it does 
> respond.  Question:  Is there any operational need or business usage 
> case that you would see that would make this setup a good idea? 
>  Because this appears to be a very risky and poor security.  I would 
> think that the SBC needs to be silently discard/drop any SIP message 
> rather than respond, as this increases the visible footprint and 
> encourages malicious actors / scanning tools.  Would think that having 
> ACLs that only permit traffic to/from the carrier's SBC would be the 
> best configuration.  Is their an opposing view?
>
> Second:  I have written some SIP software that sends malformed message 
> headers, and have noticed that the SBC responds with different errors 
> other than 403 Forbidden when headers have unexpected values.  For 
> example, when I send an INVITE with extra CRLFs, I get a 400 CSeq 
> missing header.  When I send a Contact header of "None", I get 400 
> Invalid Contact.  This leads me to believe that the SBC sip parser is 
> parsing all of the SIP message rather than always sending a 403 
> Forbidden to an IP address sourced from the untrusted public 
> Internet...this also seems to be very risky.  Is there a specific 
> security configuration with the policies that you would recommend?  It 
> seems like this introduces the risk of DoS and fuzzing attacks if the 
> SBC is parsing more of the SIP message rather than just dropping the 
> message based on invalid source IP.  Could lead to cpu and memory 
> issues if the queues are filled from invalid and fuzzed traffic.
>
> I have read the Oracle SCME Security Guide (July, 2014), and learned 
> rudimentary that there are IP ACLs, realm trust level settings, and 
> traffic queues.  But really looking for practical advice based on 
> experience with ACME.  This customer takes security very seriously, 
> and it is informative of them to see how the SBC responds, black box, 
> to attacks from the outside.  I'd like to recommend security settings. 
>  It seems like the best would be just to drop/discard any SIP message 
> from the public Internet...but wanted to get the expert's opinion on ACME.
>
> TIA, Rob
>
>
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