What happens if the swarm key from a node is stolen ?
Illegitimate nodes will be able to join the network.
If the illegitimate nodes behave honestly, they will only be able to listen to the metadata and DHT traffic as they could on the public network.
If there are a lot of them and new honest nodes don’t necessarily connect to other honest nodes, there is a risk that they carry out an eclipse attack, where they isolate their victims from the rest of the honest nodes.
If the new honest nodes are able to discover at least one honest node, they should be able to recover and discard the malicious node little by little by noticing that malicious Malory should know about awesome Alice but never talked to the victim node about her, so she is trying to hide the existence of an honest node. Of course, the more malicious coordinated nodes, the longer it takes to recover. So it is important to guarantee that nodes are connected to at least one honest node. The easiest strategy is to hard code honest node addresses (the bootstrap nodes).
TL;DR, malicious actor can try to carry out the same attacks they would on the public network.