Romanian developer Andrei Neculaesei discovered that some apps do not properly account for tel: URIs -- which pass a telephone number to the handset's dialer much like a mailto: URI would open the Mail app -- in embedded web views. Because Apple allows app developers to bypass confirmation prompts when calling the dialer from within their apps, a specially-crafted web page could cause users to initiate telephone or FaceTime calls against their will.
Tapping a malicious link from within the official Gmail app could, for example, force users to call an expensive toll number. Other popular apps affected by the oversight include Facebook Messenger and Google+.
While the issue does not represent a flaw on Apple's part, it seems likely that the company will implement changes to save developers from themselves, perhaps by altering the default behavior of such links to draw a confirmation prompt as they do when tapped in mobile Safari.
Though it is a relatively low-grade problem, it does serve to remind users that they should exercise caution when opening messages or tapping links from people that they do not know. Malware authors depend almost entirely upon consumers' lack of such basic precautions.