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.
"We have currently disabled the GetCash service while we increase the level of security required. We will enable the service again once this change is complete."
Natwest has said that it will refund any customers who have suffered losses as a result of fraud.
Dozens of customers complained to BBC Radio 4's Money Box programme that they had been defrauded using GetCash.
For example, the BBC reported that a customer from London had lost more than £950 after he found that money had been taken from his account in August, which was taken from 11 cash machine withdrawals. Each withdrawal did not exceed the £100 limit Natwest imposes per cardless cash machine transaction.
This customer had never even heard of the GetCash app service, which is offered as an additional service on the standard Natwest phone application, and the bank accused him of giving his personal details to a fraudster via a phishing email.