Apple's iOS is vulnerable to web-based attacks that force third-party apps to make phone calls and carry out other sensitive operations without first warning the user, a security researcher has warned.

Researcher Nitesh Dhanjani shows here how the planting of a simple iframe on a webpage can force the Safari browser to open Skype and dial a phone number or send a message to another Skype user. As long as Skype is installed and it stores the victim's account password, the attack will work with no warning, he wrote.

Websites could use similar techniques to force a variety of third-party iOS apps, some of which are listed here, to also carry out potentially unwanted actions without first warning the user, Dhanjani warned.

He said members of Apple's security team told him the onus is on third-party app developers to make their programs ask for permission before carrying out such actions. That didn't sit well with him.

“I feel the risk posed by how URL Schemes are handled in iOS is significant because it allows external sources to launch applications without user interaction and perform registered transactions,” Dhanjani wrote.

“Third party developers, including developers who create custom applications for enterprise use, need to realize their URL handlers can be invoked by a user landing upon a malicious website and not assume that the user authorized it. Apple also needs to step up and allow the registration of URL Schemes that can instruct Safari to throw an authorization request prior to yanking the user away into the application.”

When Dhanjani contacted Skype, he got no response. But even if the VoIP provider updated its app to seek user permission before making calls and sending messages, Dhanjani still isn't sure users would be best served.

“Third party applications can only ask for authorization after the user has already been yanked out of Safari,” he explained. “A rogue website, or a website whose client code may have been compromised by a persistent XSS, can yank the user out of the Safari browser. Since application on iOS run in full-screen mode, this can be an annoying and jarring experience for the user.”

Indeed, Safari asks for permission when encountering the tel scheme, which invokes the iPhone's default phone. But for reasons that remain unexplained Safari doesn't apply the same treatment when third-party schemes are invoked. ®