>The problem is that Anubis makes the website send out a free JavaScript program that acts like malware. A website using Anubis will respond to a request for a webpage with a free JavaScript program and not the page that was requested. If you run the JavaScript program sent through Anubis, it will do some useless computations on random numbers and keep one CPU entirely busy. It could take less than a second or over a minute. When it is done, it sends the computation results back to the website. The website will verify that the useless computation was done by looking at the results and only then give access to the originally requested page.
https://www.fsf.org/blogs/sysadmin/our-small-team-vs-millions-of-bots