Also, about this:
If I got an unknow PC in my address book, the same could happen for us, that our computers may appear in other customer's address book and they will be able to access our PCs
Please, let us know how this is supposed to happen if that "other customer" doesn't know your Host access credentials or access credentials (along with address and port) of your RU Server? From where will they get this information?
Remote Utilities doesn't employ an online "customer database". It's stand-alone software. There is no "central" database on the web unlike many of our competitors. In fact this is a big advantage of Remote Utilities vs. its "SaaS" competitors - there's simply nothing to "hack" and steal. One can only get access to someone else's Host if they know access credentials for that
specific Host.
The record in your address book appeared because:
1. You set up a custom Host installer to make it automatically send its access credentials to your RU Server address book upon installation.
2. Someone whom you do not recognize got possession of your Host package and installed it on their computer. So the host reported itself to your RU Server and was added to your Viewer address book (via the RU Server address book sync feature).
For the same to happen in your own network someone needs to either inject a custom-built Hosts to your network computers or get access to your existing Hosts somehow (for this they should know access credentials) .