[wiredfool]

As far as I can tell the security is 2 fold:

  * transactions are reversible for quite a long time. 
  * ODFIs (originating banks) are responsible for files they send.

A bad actor can get away with stuff for a while, but sooner or later, their ODFI will cut them off. Those relationships take time to build, so you don't want to be going through them quickly. And there are often deposits and other security protecting the ODFI.

So, the customer says it'sfraud. It's not their bank's problem, they punt to the ODFI. The ODFI is in the business of not taking too much risk, so they yank money from the originator. The Originator, they might be SOL, depending on if they're the one who was scammed, or if they have a way to reclaim the money.