When running a Lightning Network hub, who pays the fees when a transaction has to be broadcast to the blockchain? I've been educating myself on how the Lightning Network protocol works, and I have some questions.

Let's say that a user sends a small payment to someone and it makes multiple hops through the network before reaching the recipient. The payment then propagates backward through the chain until it reaches the payer, and everything settles out.

It's possible that a node in the middle either either intentionally or unintentionally does not pay the next node down the line. The recourse is for the node that did not get paid to broadcast the transaction to the blockchain (and close the channel?).

Who pays the fees for for that transaction? What happens if the fee is significantly larger than the amount of money the node would gain by broadcasting the transactions?

This seems like it could potentially be a large attack vector. Is there something I am misunderstanding?
It's possible that a node in the middle either either intentionally or unintentionally does not pay the next node down the line. The recourse is for the node that did not get paid to broadcast the transaction to the blockchain (and close the channel?).


No, that's not accurate. If a node in the middle of a multi-hop payment rejects a payment request, the sender simply makes another attempt along a different route omitting this node. On the other hand, if the payment contract is already established, the node is already cryptographically committed to pay. It's impossible at that point for them not to pay. A forwarding node can merely fail to collect the reimbursement for what they've already forwarded.
If a node continuously fails to forward payments, their channel partner is still free to close the channel of course, but this is for different reasons.


  Who pays the fees for for that transaction? What happens if the fee is significantly larger than the amount of money the node would gain by broadcasting the transactions?


The fee for Lightning payments is paid by the sender. They must include sufficient funds to pay fees to all nodes along the route. If the fee for closing a channel is higher than the channel's capacity, it probably makes more sense to leave it open until fees get lower.