Currently, yes, it is possible, although it cannot lead to any breach of the server or other harm. Besides, the probability that someone knows exactly at which IP address (or DNS name) and port your server service is running is very low.
However, in the near future we will implement features that will prevent such unauthorized use of the relay function. As for the nearest server update, we will add the ability to specify different communication ports for Viewers and Hosts that they use to talk to the server.
Don't hesitate to ask me if you have other questions.
Sorry for somewhat misleading previous replies. At first, we thought there was an issue with OneDrive. But the actual problem is that the program lacks the ability to load an encrypted book (a file with the .encrypted extension) to your Viewer. We also mention it in the Tips section in this documentation article.
I updated the article a bit and added a workaround, see the last item in the Tips section. In fact, if you want to use the "Dropbox" method (or OneDrive for that matter) of sharing your address book, and you want to encrypt it as well, you can simply use the Portable Viewer.
Hope that helps. Let me know if you have other questions.