The flaw has been recently discovered by developer Andrei Neculaesei from Copenhagen, and can be triggered by using the tel URL scheme.
"The tel URL scheme is used to launch the Phone app on iOS devices and initiate dialing of the specified phone number," it is explained in an Apple document.
"When a user taps a telephone link in a webpage, iOS displays an alert asking if the user really wants to dial the phone number and initiates dialing if the user accepts. When a user opens a URL with the tel scheme in a native app, iOS does not display an alert and initiates dialing without further prompting the user."
A native app can be configured to display its own alert, Neculaesei says, but unfortunately many developers have not. He checked some popular iOS apps out there - Facebook Messenger, Gmail, Google+ - and all three didn't make the necessary change and were vulnerable to the proof-of-concept attack devised by Neculaesei.
He also tested Apple's Facetime app, that apparently has a URL scheme just like tel, and can be used by attackers to see the face, and possibly discover the location and the identity of the target.
Neculaesei says that there are likely many iOS apps out there that allow this type of attack, and that its developers should configure the alerts in question.
The issue was also flagged earlier this month by information security consultant Guillaume Ross in his talk at BSidesLV 2014.