Voice Security

The Department of Homeland Security (DHS) and SecureLogix Team on TDoS Research

For those following my blog, you know that Telephony Denial of Service (TDoS) is a flood of unwanted inbound calls, typically to an enterprise contact center. The calls can arrive at any enterprise or any part of an enterprise, but are normally targeted at critical voice lines. This includes 911, other public safety numbers, hospital emergency rooms and intensive care units, key parts of financial contact centers, and other organizations.  TDoS attacks are the most significant form of voice-related DoS, because they involve malicious calls, are easy to generate, and can affect enterprises using both TDM and SIP networks. The following diagram illustrates a TDoS attack:

The Department of Homeland Security (DHS), the Federal Bureau of Investigation (FBI), and service providers have produced a number of warnings and bulletins about TDoS. A few of the more recent ones can be found in prior posts on this blog. 

There have been over 1000 attacks reported to service providers and the Department of Homeland Security (DHS). A good summary can be found at https://nakedsecurity.sophos.com/2014/01/22/tdos-extortionists-jam-phone-lines-of-public-services-including-hospitals/

Attacks such as this are simple, but still very effective. They do not involve a significant amount of volume in terms of concurrent calls and they are not very sophisticated or complex in terms of spoofing call information, such as the calling party number or ANI. We expect that in the near term, more complex attacks will be seen, involving greater sophistication in terms of spoofing call information and much greater volume. The following table illustrates the progression we have seen and expect in the future for TDoS:

In a short amount of time, we expect these attacks to become more common, be more sophisticated (complex), and involve greater volume (distributed). This will make the attacks much more difficult to detect and mitigate, both for the target enterprise as well as service providers.

The DHS Science and Technology Directorate (S&T) Cyber Security Division (CSD) recognizes the TDoS threat and has funded SecureLogix for two Research and Development (R&D) efforts.  The first effort is to define the evolving threat, define enterprise and service provider countermeasures, and build solutions for these environments. The second effort involves a broad look at security issues affecting Next Generation 911, including TDoS, which will be particularly disruptive for these environments. These R&D efforts will produce a TDoS solution that can address the most sophisticated attacks, for both TDM and SIP networks, within both enterprise and service provider networks. While the final solution is still being developed, a basic approach involves use of several filters, which score calls based upon pre-call signaling information, queries to network authentication services, and then content and possible use of turing tests. All controlled by enterprise-defined policy. These filters are shown in the following diagram:

We will be posting more information as this R&D effort progresses. You can track our progress on these efforts by following this blog and our twitter feeds at @markcollier46 and @dhsscitech.

New Telephony Denial of Service (TDoS) Tool

The article below describes a new Telephony Denial of Service (TDoS) generation tool:


As stated in the article, TDoS is a flood of inbound calls, which target a set of phones critical to business operation. The target phones (and numbers)  can be any part of a business or enterprise, but are generally those making up a public facing contact center, including those used for banking, finance, health care (emergency rooms and ICUs), government, and public safety. A TDoS attack may be of sufficient volume to overwhelm an entire business or enterprise, but can be equally effective with a smaller amount of traffic, if targeting critical resources. In this way, it is more about selecting the proper target phones and numbers (normally pulled of of public websites), timing (during the busiest part of the day and season), and complexity of the attack (spoofing the calling number), than it is about an overwhelming amount of traffic.


There are a number of ways to generate TDoS attacks, including use of SIP trunks and free PBX software such as Asterisk, possibly using Skype as referenced in the article, or using a tool like the one described in the article. The advantage of a tool such as this is:

  •  It can generate a sufficient number of concurrent calls to overwhelm a  small or moderately  sized target.
  • Is turnkey and easier to set up than a SIP trunk and Asterisk.
  •  Can generate a complex attack (assuming that it can indeed spoof the calling number for all calls).
  • Is anonymous and hard to track. It can be used anywhere where there is cellular coverage.
  • Is difficult for a service provider to shut down, because the calls are coming in through the cellular network

The last point is significant, because this means of originating TDoS calls is more difficult for the service provider to isolate, than say many calls coming from a single SIP trunking provider.

The TDoS attacks enabled by this tool can be used purely for disruption, as a threat to enable extortion, or to flood a victim with calls (or texts) to prevent authentication calls from the victim’s bank.

Telephony Denial of Service (TDoS) Service

There have been several advertisements for Telephony Denial of Service (TDoS) attack services popping up. I provided a link to one below. These seem to come and go, as they are removed from sites, but this one has been up for a while. The service is very cheap, $70 for week, which if targeted towards a hospital emergency room, Intensive Care Unit (ICU), public safety site, or any small business, where there are a handful of critical phones and attendants, this service would be very disruptive. Of course there are other ways to do this yourself - using Asterisk and SIP trunking, but this is easier for a non-technical attacker.

They even offer a 10 minute free trial :)

Since I saw the service, it has been enhanced to state that the calls can be made with different source numbers. I don't know how sophisticated this is - are they random, legit numbers, etc., but of course this makes an attack much harder to deal with.

It isn't  clear what the flood capacity is. it says the interval between calls is 1-3 seconds. The calls are automatically generated. You have the option of playing an audio file, but that costs more (requires the attacker to generate RTP).


If anyone tries it out, please let me know the results.

Report on Fraud - Voice Has Become The Preferred Medium for Attack

Here is an interesting report on a variety of fraud issues. One thing that struck me is that voice has become the preferred channel for fraud. Voice SPAM, scams, vishing, social engineering into contact centers, etc. Voice used to be the most trusted communications medium, but now it has become the LEAST trusted. Public voice has a ton of issues - is it any wonder that users are moving to closed systems for voice and messaging???