Groundhog Day: Don’t Pay Money for Some Else’s Calls, Still

Hi everyone,
it’s me again with another story of a toll fraud incident at one of our customers (not the same as the last time of course ;-)).
The story began basically like the last one: We received a call with an urgent request to help investigating a toll fraud issue. Like the last time I visited the site in order to get an idea on what was going on exactly. The customer has a VoIP deployment consisting of the whole UC Suite Cisco offers: Call Manager, Unity Connection for the voice mailboxes, Cisco based Voice-Gateways and of course, IP phones.

During the initial meeting I was told that the incident had taken place over the weekend, and had caused a bill of almost 100.000€ during this time period. Similar to the other incident, described two weeks ago , our customer didn’t discover it by himself but again the Telco contacted him beacause of that high bill. After the meeting I got ready to work my way through a whole bunch of log- and configuration files to analyze the situation. Spending 1 ½ days on the customer site to analyze the issue, I was able to reconstruct the incident. As stated earlier, the customer uses Cisco Unity Connection as voice mail application. Unity is reachable over a specific telephone number so that employees are able to listen to voice mail messages if they are on the road . When dialing this specific number, one has to enter the internal extension followed by a PIN for authentication. It turned out, that someone had brute forced one of the mailboxes PIN.

So how could this toll fraud issue happen by just bruteforcing the PIN of a mailbox? After successful authentication though the PIN, one is also able to configure a transfer of a call to a telephone number of your choice. Now it should become clear, where this is going…
After the bad guys retrieved the valid PIN, they configured a call transfer to some $EXPENSIVE_LONG_DISTANCE_CALL. In addition they changed the PIN in order to access the system whenever needed. As the issue started on a Friday evening (when almost everybody had already left for the weekend) nobody noticed the compromise of the mailbox. The bad guys logged in about 200 times during the weekend and configured different numbers to which the calls should be transferred. They started with some numbers located in African countries, which wasn’t successful because the configuration of the Call Manager blocked outgoing calls to such suspicious countries.

So, how could they initiate the calls nevertheless? These guys were smart. After realizing that the first approach wasn’t working they found a clever way to circumvent the restriction. They just used a so called “Call-by-Call” Provider. To use such a provider you have to prepend a provider specific prefix to the number. E.g. one prefix of a German provider is 010049. So they dialed 010049+$EXPENSIVE_LONG_DISTANCE_NUMBER and were able to circumvent the restriction on the Cisco Call Manager.

The first question which came to my mind was: Why can Cisco Unity initiate outbound calls? Well, according to our customer, there were some requirements that Unity should contact some home workers on their normal phone that new messages are present. In order to stop the potential exploit on short notice, we first configured the Call Manager denying Unity to initiate outbound calls. After digging into the configuration of Unity Connection and the Call Manager I found some configuration on the Unity connection box which enabled the attacker an easy game.

1. The PIN was only 4 digits long.
2. Unity Connection did not prevent the use of trivial PINs like „0000“ or „1234“.
3. There was no restriction on to which number a call transfer could be configured.
4. The ability to configure a call transfer over the Phone Interface is at least debatable.

These properties are a little unfortunate as Unity connection gives you all the tools you need to address the issues mentioned above. However, in this scenario the config had not been handled appropriately. So this case could basically be broken down to configuration weaknesses which favored the attacker to exploit the issue. Like in the last incident , the initial deployment and configuration was done by an external Service Provider.

So how can we assure that this won’t happen again?

1. Use longer PINs. I recommended that the PIN should be at least 6 digits, which increases the number range you would have to bruteforce significantly, causing the attacker requiring up to 100 times as long for the attack! The password policy for the mailbox is configured in a so called authentication rule, where one can define all sort of things as for the mailbox password. In this authentication rule it was just one click to disable the use of trivial PINs.
2. In Unity Connection, one can configure so called restriction tables to define to which numbers a call can be transferred. In the default installation there are some predefined restrictions, which didn’t work with the number plan of this particular customer.
3. I recommended evaluating the need for configuration of call transfers over the phone, along with the advice to disable this functionality if not necessary.

All in all it is not rocket science to configure Unity Connection in a secure way, which unfortunately doesn’t mean you won’t find all kinds of scary misconfigurations. All the years at ERNW showed me this impressively.

As already said : It can cost you quite a lot of money if you do not take precautions to prevent that kind of incidents in the first place. So if you own the mentioned products (or plan on integrating them in your environment) check the configuration to ensure something like this won’t happen to you 😉

And one more thing: If you are interested in more VoIP security coverage don’t miss out Troopers 2012 where Enno and Daniel will give a talk on how to compromise the Cisco VoIP Crypto Ecosystem.

Have a great day,
Chris