FanBox is very safe. I have never faced any security issue over there. I am active there for more than 2 years now and earning regularly. Even in payments they are very safe and secured. I always get my payments safely via PayPal.
If the question about technical safety of your machine then there is not such thing that can infect your machine. They don't provide stuffs to download like software, media files etc. That is why it is all safe and secured.