Anyone can stand up their own instance, subscribe to remote communities, and start receiving all the data necessary to show those communities. That includes posts, comments, and votes too.

Every instance operator is in control of a database containing all the activity for communities that instance’s users are subscribed to. They can do whatever they like with that data. That’s a consequence of how federation works.

The protocol as it stands today is also generally vulnerable to any malicious instance. A malicious Lemmy server could emit spam, send out bogus votes, or alter its users’ comments after the fact (ahem, spez) and disseminate the modified versions. The main tool that other instances have to deal with a malicious instance is … yup, defederating.