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

Robert Nystrom ronystrom at gmail.com
Wed Sep 3 11:49:44 EDT 2014

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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