iTunes Store. The problem is, that a mobile app developer has proved that this pop-up could be generated by an app that is simply attempting to capture your Apple ID password.
This appears to be the first risk that has surpassed Apple’s rigid App Store security. A seemingly legitimate app could generate this pop-up. One way to tell if this is a legitimate prompt for your Apple ID password is to press the home button on your phone. If the prompt disappears, it could be an attempt to steal your password. If the prompt remains on screen, it’s a legitimate prompt. Another recommendation is to close this pop-up, go to the Settings app and to your Apple account and enter your password there, if prompted.
I’ll blog on this further if more details come to light.