Is the Lightning Network susceptible to man in the middle attacks? 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?
Such an attack is not limited to just Lightning; it could happen with just normal transactions and normal Bitcoin usage. However performing such an attack is very expensive and unlikely. It would require A to control as much hashrate as the entire network has. Otherwise blocks will come slowly for B and eventually warn him if too few blocks are received within a given time frame. Furthermore, if blocks were invalid, B will ban A's nodes and find other nodes to connect to. A would also have to control B's internet connection in order to prevent him from connecting to other people and opening channels with people other than A.

Lastly, the lightning network has a minimum balance requirement (minimum must be kept for both parties after it has been met, but both parties do not need to start with the minimum in the channel) so B would not lose all of his money. There is also currently a maximum amount that can be put in a channel so not a lot of money is risked.