There have appeared hundreds of communities on my instance I have never searched. Where do they come from? Is it some spam attack? How do I prevent this, and how do I get rid of them?
Well, it says that I have 11 users on my instance. And I don't mind having them if they are not bots or communities crawlers. But it's hard for me to believe that 11 users could have searched so many communities in such a short period of time.
Is there a way to check that they're legit users?
Not sure how to check if they're bots or not, but I would enable captcha for registrations. Also that's where I was pointing at, other users might have searched for these communities. I also have many of them on my instance which I've never searched for, and I think that it's all from them. All it takes is for some random user to click on a community in a post and it's then added to your instances.
Uh yes... There are instances (mine included) that'll automatically defederate you if you have too many users with too few interactions, so better enable captcha if you're worried about bots.