Exploring the Telephony Denial of Service (TDoS) Q&A

Pierluigi Paganini January 30, 2014

Interviewed with Mark Collier, CTO and VP of Engineering at Securelogix to better understand the topic of Telephony Denial of Service (TDoS).

I briefly interviewed Mark Collier, CTO and VP of Engineering at SecureLogix (Www.securelogix.com) on the topic of Telephony Denial of Service (TDoS). Mark just completed the Hacking Exposed: UC and VoIP book, which covers TDoS and related topics. Mark has been working in this space for some 20 years, maintains the www.voipsecurityblog.com blog on this topic, and is a frequent speaker at conferences on this topic.

Hi Mark, I have seen a lot of press on Telephony Denial of Service (TDoS). What is it?

Telephony Denial of Service or TDoS is a flood of unwanted, malicious inbound calls. The calls are usually into a contact center or other part of an enterprise, which depends heavily on voice service. Examples are financial contact centers, Intensive Care Units (ICUs) and emergency rooms in hospitals, and public services such as 911. The intent of the attacker is disruption or financial gain through extortion.


As far as the press is concerned, let me add a couple of articles on the topic that could be interesting for my readers:

Can you give some examples of real world attacks?

The “Payday loan SCAM” is probably the best known. In this scam, the attacker obtained a list of individuals who have taken out short-term “payday” loans. The attacker calls the victim’s place of business, states that the individual has not paid their loan (which is often true), and that if they don’t pay, the attacker will embarrass and harass them by repeatedly calling their place of business. This attack has progressed to where the attackers are also just calling ICUs and emergency rooms and making the same threat. When the victim or enterprise refuses to pay, which is what they should do, they are hit with a barrage of calls. The calls are generated through a combination of cheap labor pools and automated call generation. Amazingly, over $5,000,000 has been paid out on this scam. The attacker provides a pre-paid, anonymous debit card, upon which the victim adds funds.

There have been a number of attacks and press information about attacks against emergency services. Some intentional and some unintentional (a famous rapper sent a tweet to his 600,000 followers to call in complaints to the local sheriffs office, when bled over to the emergency services). We have also worked with multiple financial enterprises, whose contact centers were victims of TDoS or call pumping attacks (which created a TDoS condition).

What makes these attacks possible?

It is very easy to anonymously generate calls into the public voice network. It is very easy to spoof your calling number, so the victims don’t know where the calls are coming from. Attackers can use cheap labor pools and hard-to-trace VoIP applications such as Skype to generate the calls. More likely though, the attackers can use a variety of automated techniques to generate the calls. With inexpensive SIP access to the network, free and powerful IP PBX software like Asterisk, call generators, the ability to spoof the calling number, and ready information on the Internet, make it easy to generate automated TDoS. The Hacking Exposed: UC and VoIP book covers this process in detail, including a sample attack. Turnkey TDoS tools such as those discussed by Dancho Danchev can also be used.

What do you recommend enterprises do about these attacks?

The right place in an enterprise to address these threats is on the perimeter, before they saturate the trunks and overwhelm the internal IP PBX or contact center system. Keep in mind that most of the attacks enter an enterprise through TDM trunks. While SIP trunks are being deployed rapidly, they account for perhaps 10-15% of the overall trunks, so any solution has to address both. IP PBXs have some basic capability, but it is very limited. Session Border Controllers (SBCs) can provide some mitigation, but they only work for SIP and really only look at the underlying packets, not the malicious calls. Companies like SecureLogix offer application-level security solutions, which monitor all types of trunks, and look at all the signaling and media, to identify the calls that are part of the TDoS attack and remove them from the network.

Pierluigi Paganini

(Security Affairs –  TDOScybercrime)

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