Security researchers at SPI Labs are warning iPhone users not to use a special feature that lets them dial telephone numbers over the web using the iPhone's Safari browser.

The feature was created to give iPhone users a simple way to dial phone numbers listed on webpages, but according to SPI, the feature could be misused.

Attackers could exploit a bug in this feature to trick a victim into making phone calls to expensive "900" numbers or even keep track of phone calls made by the victim over the web, said Billy Hoffman, lead researcher with SPI Labs. The iPhone could even be stopped from dialing out, or set to dial out endlessly, he said.

"Because this vulnerability can be launched from websites, everybody who has an iPhone has the potential to get exploited," Hoffman said

In order for the attack to work, the bad guys would have to either trick iPhone users into visiting a malicious site or make a legitimate site send untrustworthy information to the iPhone using what's known as a cross-site scripting attack. "Any time someone could control the content that's getting sent to the iPhone [the possibility of an attack] exists," Hoffman said.

SPI is not releasing detailed information on how the web dialing feature could be exploited, but the company contacted Apple on 6 July and is working with the iPhone maker to prevent these types of attacks, Hoffman said. Apple could not be reached immediately for comment.

Because Apple is encouraging software developers to write web applications for the iPhone that use Safari, the browser has come under particular scrutiny from iPhone hackers.

Researchers had previously noted that Safari could be used to misdial numbers, but Hoffman's post suggests that this could be done more easily than previously thought.

Not everybody thought the SPI findings were groundbreaking, including Dave Aitel, chief technology officer with Immunity. "If you can make calls from the browser, you can make fake calls from browser," he said via instant message.

So should iPhone users stop using the web to place calls? "Yes," said Aitel. "If they know a lot of hackers and are a special target."