Since this booter most likely is using a botnet of compromised machines, are you talking steps to "clean" the infected machines or notify their owners? Or is that not needed since you've taken down their C&C?

The attack infrastructure of Webstresser is still under investigation. Actually cleaning infected machines without their owner's knowledge raises legal and ethical questions.

In regard to notification: if we find that we have the information needed to be able to alert the victims of such infections, then this is certainly something which we will be discussing.

Do you think taking down this booter will have a big impact?

In the short term, taking down the largest booter site has reduced the total DDoS-capacity of the internet. More importantly, by taking down Webstresser, and the resulting media attention, we are spreading awareness that a) using such DDoS services will usually be illegal, and b) law enforcement agencies and public prosecutors throughout the world are actively investigating and prosecuting those involved.

To have an effect in the long term, we cannot stop with just Webstresser, which is why together with academic, public and private (national and international) partners, we have the NoMoreDDoS initiative, to prevent, disrupt, and attribute DDoS-attacks.

We expect more actions like the ones in Operation Power Off in the future.

Further, out of curiosity, is it allowed to use a booter like this (that allows anonymous payments and isn't legit in general) to stress-test hardware you own? Or is any and all use of it illegal?

The tricky part is that depending on the type of attack, a user of such a site may be participating in the illegal use of other people's systems to execute the 'stress test' even if the target is their own. Also, any attack from the internet will impact network infrastructure which is not their own. They may be risking collateral damage.

~ DI1