Citigroup customers who do mobile banking on an iPhone should head to the Apple App Store immediately for an upgrade. A flaw in the Citigroup mobile banking iPhone app released in March 2009 causes personal information to be saved in a hidden file on the mobile device, the banking giant revealed in a letter to customers dated July 20, a day after it released an upgraded application. Without the upgrade, customers' personal data -- including account numbers, bill payment information and access codes -- is saved on the iPhone.