SPI Labs says iPhone users should not use its feature that lets them dial telephone numbers over the web.

The feature was created to give iPhone users a simple way to dial phone numbers listed on web pages, 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 dialling out, or set to dial out endlessly, he said.

"Because this vulnerability can be launched from web sites, 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 web site or make a legitimate web site send untrustworthy information using 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 dialling feature could be exploited, but the company has been working with Apple since 6 July to prevent these 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 ground-breaking, including Dave Aitel, chief technology officer with Immunity Inc. "If you can make calls from the web browser, you can make fake calls from the Web 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."