Kind of, but if you control the card reader you could charge more for the transaction without showing the amount, for instance. And maybe even send the money to a different account.
Banks don’t have wide open protocols where anyone can submit a credit card transaction and have it go to arbitrary accounts.
Remember that credit card companies eat the cost of the fraudulent charges. They’re not going to make it easy for those to occur.
If someone altered the terminal to charge more (a hypothetical suggestion above), they could recoup that money from the merchant’s account because they have an agreement in place.
You can’t run arbitrary transactions to arbitrary accounts (the other proposed attack above) because there isn’t an agreement in place.
I've got a couple on my desk right now; there's nothing I can do with them to steal money, even though they're in dev-mode.
That would be something I'd like to see. The terminals I have worked on do not store merchant details. A merchant ID is stored on the terminal, with the ID mapping to an actual merchant account on the backend.
In order to have the merchant account on the backend, you need to be a customer of that terminal supplier. If you are a customer, they know:
a) Where your terminals are deployed
b) Your real details
c) The terminal's ID and the terminal's serial that maps to that specific merchant ID.
So, let's say we do change the merchant ID from `12` to `24` on the terminal. The request goes up with `transaction(<amt>, 24, 'sr-12345')`, and then the backend rejects because terminal sr-12345 is not mapped to merchant 24.
Lets say we also manage to fake the serial number. Then the transaction is approved, but can be easily reversed because merchant 24 is a customer and we have:
1. Their bank account number 2. Their physical address 3. The company registration number 4. Verified ID copies of the owner, directors, managers, etc. 5. Their money (from their transactions).
So, yeah, I'd love to know more about how they execute this hack; it would require complicity on the backend to a large degree.
3.
Probably not worth the small amounts you could make before caught
Not supposed to be possible on a certified terminal. The certification tests this particular case (the transaction is a hash of the keys, amount and a few other things. The display of the swipe/tap/insert screen and the pin-entry are under control of the certified kernel, so the userspacve application has no control of the amount that is displayed).
> And maybe even send the money to a different account.
Not from the card reader.