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,


Continue reading

Don’t Pay Money for Someone Else’s Calls, Again

One of our customers called us recently and asked for some support in investigating a toll fraud issue they encountered in one of their sites. Their telecommunications provider had contacted them informing them that they had accumulated a bill of 30.000€ over the last ten days.

Without knowing anything more specific, I drove to the affected site to get the whole picture.

They have a VoIP deployment based on Cisco Unified Communications Manager (CUCM, aka Call Manager) as Call Agent. The CUCM is connected via a H.323 trunk to a Cisco 2911 ISR G2 which is acting as a voice gateway. The ISR has a primary rate ISDN (PRI) Interface which is connected to the PBX of the telco. Furthermore they use a feature called Direct-inward Dial (DID) or Direct Dial-in (DDI) which is offered by Telco’s to enable calling parties to dial directly to an extension on a PBX or voice gateway.

Basically one then has a so called head number (in networking terms a prefix), together with some phone extensions. When someone from outside wants to call, he dials the head number + phone extension. Before the telco forwards the call to the ISR, the head number is stripped and only the phone extension number is forwarded to the voice gateway. E.g. when calling 12345-678, the local voice gateway will only see the 678 as called number.

After having a good overview of the design, I started to dig around in the log and configuration files to understand what exactly happened and why.

So here is what happened:

Apparently someone from some East European country had called the head number of our customer and prepended a “malicious number” (in some country in Africa) to which the ISR should setup a call.  The ISR only sees the malicious (African) number because, as said before, the head number was stripped by the telco. The malicious number was of course some $EXPENSIVE_LONG_DISTANCE_CALL ;). So the voice gateway received a call from the PBX and forwarded it back to setup the call with that number.

Before we proceed, a little bit of theory how a Cisco router decides how to forward a call, might be helpful:

In Cisco IOS, the call-routing table is configured based on so called dial-peers. These dial-peers specify how a call with a specific destination number should be forwarded.

As an example:

dial-peer voice 1234 pots
description ===incoming_calls===
incoming called number ^[2-7]..$
port 0/3/0


This configuration tells the router that calls to a number which matches the regular expression, should be forwarded to port 0/3/0.

As it turns out our customer uses the following dial-peer which is used for outbound calls.

dial-peer voice 5678 pots
description ===outgoing calls===
destination pattern 8T
port 1/1/1:15 -> The ISDN Interface


The T is a placeholder value which means that any amount of digits can follow the 8. The reason the pattern matches the digit 8 is that this digit must be dialed before the actual number.

Do I have to mention that the malicious number also starts with an 8? 😉

So back to the presumed course of action:

The call with the malicious number hits the router. The router tries to match a configured dial-peer to forward the call.  I think you can guess which dial peer matched for the malicious number 😉

So the router sends the call back to the PBX to setup a call to the malicious number. Which is billed to our customer…

We then monitored the situation and applied a workaround (more on this in a minute) and observed what happened. As it turned out, unfortunately the attacker was able to circumvent our workaround. We discovered that is was possible to “dial-in” to the router directly by just calling the head number (as the PBX leaves the called number field empty). E.g. the called number field in the log files looks like this:

“Called Number=”


The router subsequently provided a line and it was possible to call the number again. Our workaround did only affect incoming calls with the number prepended, but not those where the router is the actual origin of the establishment of the call.

So how can we resolve this issue and stop the toll fraud?

As a long-term solution the configured dial patterns should be reviewed and modified to prevent such things in the future, but – given the overall complexity of the setup – this could not be done overnight.

I am currently working with the customer to develop more suitable dial patterns. I will write a follow up post with the final results when we are finished.

In the mean-time, we developed a temporary workaround to prevent this from happening again:

In Cisco IOS you can manipulate the calling or called-number with so called translation rules and you are also able to reject calls based on the called number. Our customer does not use any extension beginning with 8, so we can drop all calls on the gateway which starts with 8 as called number. So we developed the following translation-rule:

 voice translation Rule 11
  rule 1 reject /^8+/
  rule 2 reject /^$/
 voice translation-profile reject_calls
   translate called 11


Rule number 2 addresses the case when the called number field is empty. We mapped this profile to the dial-peers responsible for the incoming calls and specified that calls with the numbers in the translation rule must be rejected.

dial-peer voice 3456 pots
description ===Incoming_Calls===
call-block translation-profile incoming reject_calls
call-block disconnect-cause incoming call-reject
incoming called-number

Lessons learned:

Be careful when you develop and implement your dial patterns, as errors in this space can cost you quite a lot of money 😉

VoIP is a complex technology and this complexity can lead to all types of vulnerabilities, as Daniel and Enno are going to show in their talk at Troopers 2012. Toll fraud is still quite common and happens all the time, as you can see in an ERNW newsletter from 2009 covering a similar story from another environment.

On a side note:

The telco told us that our customer is the 8th customer affected by a toll fraud issue in the last two months. According to the telco all eight companies are in the same city, and the initial VoIP deployment at our customer was performed by an external service provider.

Maybe the same service provider has done the deployment in the other companies too…

Have a great day,

Continue reading