The app, which allows customers to draw money from a cashpoint without a card, was suspended at the weekend and Natwest initially claimed that it was simply carrying out a planned update.
Natwest is a subsidiary of Royal Bank of Scotland, after a botched upgrade that was made to batch processing software CA 7 from CA Technologies.
When the bank first suffered the major meltdown in June, were frozen to use its GetCash service to withdraw money from their accounts.
RBS has now admitted that the application isn't currently 100 percent secure.
A spokeswoman for the bank said: "We are aware of an increase in fraudsters targeting customers with phishing attacks to gain access to the GetCash service.