A security precaution skipped in mobile applications such as Facebook's Messenger could be abused to make an expensive phone call at a victim's expense, a developer contends.
Phone numbers often appear as links on a mobile device. That is possible by using a Uniform Resource Identifier (URI) scheme called "tel" to trigger a call.
URI schemes are a large family of descriptions that can tell a computer where to go for a certain resource, such as launching a mail application when an email address is clicked.
Andrei Neculaesei, a full-stack developer with the wireless streaming company Airtame in Copenhagen, contends there's a risk in how most native mobile applications handle phone numbers.
If a person clicks on a phone number within Apple's mobile Safari browser, a pop-up asks if a person wants to proceed with a call.
But many native mobile applications, including Facebook's Messenger and Google's +, will go ahead and make the call without asking, Neculaesei wrote on his blog.
Mobile apps can be configured to display a warning, but on most applications it's turned off, Neculaesei said via email on Thursday.
He found a malicious way to abuse the behavior. He created a Web page containing JavaScript that caused a mobile application to trigger a call after someone merely viewed the page. The JavaScript automatically launches the phone number's URI when the page is opened.
A demonstration on his blog showed how a malicious link, sent through Facebook's Messenger, will launch a call when viewed. Neculaesei wrote that someone could create a link that when viewed immediately launches a call to a premium-rate number, which the attacker gets the revenue from.
His testing found that Facebook's Messenger app, Apple's Facetime, Google's Gmail and Google + applications do not warn users before launching a call.
A Facebook spokeswoman said Friday its mobile application would be updated soon to fix the issue. Google couldn't be immediately reached for comment.
Neculaesei wrote that he only tested a few big-name apps, but it's probable that smaller teams and platforms haven't thought about the risk either.
Neculaesei's finding dovetails with research presented earlier this month at the Bsides security conference in Las Vegas.
Guillaume K. Ross, an information security consultant in Montreal, found that URI schemes can be abused, resulting in data losses or compromising a person's privacy. A video of his presentation is online.
