It seems like a man in the middle attack could be performed in a Lightning Network.
Let's say A opens a channel with B. If A manages to control all the connects B has to the bitcoin network (becoming a man in the middle), A can prevent B's attempts to propagate or publish refund transactions. In this case, A can send some money through B to another address A controls, then publish the original balance transactions. If A can continue to MITM B for the 10 days (or whatever) it takes for that transaction to go through, A would have successfully stolen all the funds A was able to convince B to transfer.
B could potentially become aware that either the blockchain data he's getting isn't valid or simply become aware the balance transaction has been published, but if he can't get on the bitcoin network to publish the refund, he's screwed.
Is this scenario possible?