DDoS, or distributed denial-of-service attack, is a cyber-attack which disables a network for intended users by flooding the target with many different sources in order to overload the system. There are a lot of DDoS software tools available on the internet, many of which are quite user-friendly. Still, DDoS attacks are extremely dangerous and may cause the loss of millions of dollars, depending on the duration of the attack. Of course, there are a lot of ways to protect against a DDoS attack, such as firewalls, blackhole routing, upstream filtering and others. Bitcoin has some built-in protections against DDoS attacks, but is still vulnerable and may be slowed down by them. Cryptocurrency exchanges and various web-sites of concrete currency may be attacked, made unavailable for users and even be robbed for some amount of tokens while being under a DDoS attack.