I believe GP is confused.

You can read a card (stripe) with one of those cheap readers. The magnetic stripe is not encrypted and you can extract the card number (PAN), expiration date, and cardholder name. Some other less important bits too. You cannot extract the CVV this way, but that is not required for transactions.

Most cards are EMV now. That data is encrypted and it's read/write. These keys can leak, putting you back into a similar state as if you'd read the card via magnetic stripe.

But no amount of card reading gives you the ability to submit transactions to the network. For that, you'd need merchant credentials for their gateway or processor, and you'd usually need to have a presence on the merchant's network (to get through the upstream firewall).

These are all achievable things. Doing so gets you the ability to create transactions against the card. These transactions will be submitted to the network and approved or declined. If approved, funds will settle from the issuing bank to the merchant bank, possibly in multiple steps.

I'm oversimplifying a bit, but the essential point is that funds will settle to the merchant's bank account, not yours. You can cause some headaches, but you cannot steal money.

The compromise of a credit card terminal is only interesting because it gives an attacker the ability to steal the card details for all cards that are subsequently used at the compromised terminal. They can be saved and retrieved later, or sent out to a C&C server, etc. Then these card details can be used for all the usual types of credit card fraud.

Wouldn't the concern being redirecting the money to a different merchant account? Of course that would mean you are easily tracked down when found out but I'm sure you can find a way that some schmuck who doesn't actually know anything about you ends up with that role.

Then again, changing the merchant account is usually only protected by a numerical PIN so you wouldn't need root access. Maybe it would be to send the original requested amount to the expected merchant account but also do a separate smaller transaction to your own account?

The configuration of the settlement bank account happens at the processor. If you want to change it, you need to talk to customer service and fill out a PDF form, with signatures and other human verification processes.

If it were possible to change the settlement account via an online portal or similar, then you'd need the user login credentials for that portal. In which case, compromising the card reader has no additional value.

this is all true, but if you've compromised a merchant deeply, it doesn't seems impossible to run a $1 charge as a (bad) customer and then give that customer (you) a, say, $1,000 refund.

You can only void existing transactions, so the money would be returned to the same card, and the amount would be limited to the originally captured amount.

It is possible to create a "push" transaction too of course. Visa Direct, Mastercard MoneySend, etc. But that requires a separate merchant account, and should not be possible from the card reader or POS.

If you've compromised deeply enough to be in the AP system, you can create arbitrary payments, but that's well out of scope for this thread.

> if you've compromised a merchant deeply

Then you could just complete a normal transaction on their website and introduce your account in to their system that way, no real need for a compromised terminal?