What is a Telephony Denial of Service (TDoS) attack?

Telephony Denial of Service (TDoS) attacks have one goal: to overwhelm a service with so much traffic that the system collapses under the pressure. This type of cyber attack is especially concerning because it isolates and endangers the lives of victims.


In coordinated attacks, TDoS can cripple public infrastructure and prevent people from calling emergency services or alerting their banks to suspicious activity on their accounts. Your telephone company is responsible for a system that is resistant to TDoS attacks. Still, there are some things you can do in the event of a TDoS attack.


What is a Telephony Denial of Service (TDoS) attack?

TDoS are attacks in which hackers use automated settings to flood a telephone service with a large amount of fake traffic, preventing legitimate calls from going through. To put it in perspective, think of this type of Denial of Service (DoS) attack as using an army of toy cars to occupy a highway and create traffic jams that prevent real cars from driving down that road.

TDoS attacks are not new. In fact, there have been enough recurring problems for federal security agencies to consider them a threat to public safety.

How does a TDoS work?

The setups of TDoS attacks range from simple to complex. A common feature, however, is that these attacks primarily target telephone systems based on Voice over Internet Protocol (VoIP) technology.

In simple TDoS attacks, the attacker uses an automated phone dialer or software to generate and launch a barrage of automated calls. This type of attack doesn’t require much to execute: a Linux server on an old computer and robocall software is all the attacker needs. Due to its simplicity, such attacks typically target individuals and small local businesses.

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The setup for complex TDoS attacks is more extensive and requires some planning and execution from the attacker. Typically, an attacker uses an army of botnets to perform a distributed attack. Of course, one way to get this arsenal is to buy a botnet on the dark web. Another approach is to spread malware online and claim real people’s smartphones. Attackers prefer the latter because it is easier to bypass call filters and spam blockers.

What is the impact of TDoS attacks?

TDoS attacks close the lines and prevent victims from receiving or receiving calls. The impact for an individual is that they cannot call emergency services – or even anyone at all. If it’s part of a complex cyber-attack scheme, a victim could alert their banks to fraudulent activity on their accounts.

Likewise, the consequences of TDoS attacks for companies and municipalities are financial. Typically, attackers prefer to use TDoS attacks to hold businesses and municipalities for ransom. Often the victim had no choice but to comply with the attacker’s demands. After all, resistance would mean losing productive man-hours, customers or, in the case of a municipality, civil unrest due to broken public infrastructure.

How to prevent TDoS attacks

The task of preventing TDoS attacks lies with your telephone service provider and government agencies. Your telephone service provider makes the communications infrastructure resistant to TDoS attacks, usually by implementing security protocols that filter and block calls from known attackers. Government agencies such as the Federal Communication Commission, the Department of Homeland Securityand the FBI take it upon themselves to set rules, investigate, and prosecute attackers.

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Use a reputable phone service provider

For your part, finding a reputable phone service provider is all you need to do. And if your current carrier doesn’t offer solutions against spoofing or call screening, consider switching to one that does. In addition, talk to someone from the telephone company and have them send a technician to configure your communication settings to make them resistant to TDoS attacks.

Install a Session Border Controller

Meanwhile, if you have the technical know-how, install a Session Border Controller (SCB) to enhance the security of your phone system. SBCs act like firewalls that manage call requests. So when the controller detects unusual traffic that could cripple your system, it blocks those connections. You must refer to your telephone system manuals to configure your SBC. Your phone service provider should have this guide online if you have lost your hard copy.

Should you be concerned about TDoS attacks?

Not really. Worrying about a TDoS attack is primarily the responsibility of your telephone provider and the municipality. Still, you are right to take steps to prevent such attacks, especially if you know a small business or live in an area where TDoS attacks are common or expected. In addition, we recommend that you take steps to reduce your exposure to more common cyber-attacks, such as corporate email compromise attacks.

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