The cables use light that stays within the cables which makes it difficult for hackers to access your data. While this is the case, it doesn’t mean that it’s impossible for hackers to access your information.
All the hackers need to do is to have a network tap and a physical access to your cable. Due to this risk, you should take the safety of your computers seriously to prevent people from getting into your network. You should also encrypt any data that you want to be kept private.