Regulators have also launched investigations into security measures at the affected firms, and the credit card firms have said they will cover any financial losses.
The cybercrooks charge only $0.25 to $9 per transaction and escape detection largely because consumers took the loss rather than going through the hassle of disputing purchases with their credit card firm.
There were also fresh signs of pessimism among the country's manufacturers in the run-up to the referendum, but a leading credit card firm said spending rose immediately after the vote.