Is a physical private network directly connected between hosts secure?

Is a physical private network directly connected between hosts secure?

Let's consider host A and host B with network interfaces connected directly with an ethernet cable. It can also be assumed that the physical security of this cable is okay.

Neither host supports routing from other interfaces on the host.

Is a man-in-the-middle attack on the connection between A and B even possible, and would authentication be needed at all?

Edit: Clarifications: The hosts are embedded style devices where routing can be disabled at installation and only enabled again by an attacker via total host compromise. The network cable runs in a physically secure environment.