It really depends on the type of integration used between the terminal and the POS. Sometimes the terminal handles everything and just communicates the status t

It really depends on the type of integration used between the terminal and the POS. Sometimes the terminal handles everything and just communicates the status to the POS, in which case your attack would be viable. But it's also possible for the terminal to just package up the info and pass it off to the POS to handle the communication. Source: I've built a custom integration with Verifone terminals.