the answer to your question is very extensive and complex (but very!). In my book "Technology of IP NETWORKS" (in german) in the chapter "18.7.6: 5G Network Security" you will find many literature sources. These can help you - see:
Hamza Ijaz this more like a "how long is a piece of string question". 5G brings with it the premise of ubiquitous connectivity and media independence, in addition to increased speed ...etc. With that in mind, classic (D)DoS attacks, prolific in high-speed fixed networks, will now be ever more applicable in the 5G space.
Then there is the stack of privacy/authenticity/device security questions, and a whole load of different angles to look at on top of the views expressed both in this reply and others who've contributed to the discussion.
I hope this kind of gives you a pointer as to where to more specifically direct your question.