How to crash and restart an iPhone with a CSS-based web attack

A security researcher has revealed a method of crashing and restarting iPhones and iPads, with just a few lines of code that could be added to any webpage.

Sabri Haddouche tweeted a link to webpage containing his 15-line proof-of-concept attack, which exploits a vulnerability in the WebKit web rendering engine used by Apple’s Safari browser.

Haddouche, who for a day job works as part of Wire’s security team, demonstrated that the Safari browser could be easily overloaded by applying a CSS background-filter property to over nested 3,000 <div> tags.

As the WebKit’s rendering engine consumes resources, iOS eventually freezes and devices can crash and restart.

The good news is that the weakness can not be exploited to steal information from iPhone and iPad users. However, it could be used by a mischief-maker or malicious attacker in a “denial-of-service” type of attack, effectively stopping a device from working.

Many users would certainly find it a more than trivial inconvenience to have their smartphones power cycle off, and take a few seconds to restart again (requiring a passcode to be entered).

pic.twitter.com/jhzIPIeo2Y

— Sabri (@pwnsdx) September 16, 2018

According to reports, the attack works on a variety of versions of iOS, including the latest iOS 12 beta.

But it’s not just iOS users that are potentially at risk.

For instance, Some have even produced videos which appear to demonstrate that Apple Watches are also vulnerable.

Furthermore, Haddouche told ZDNet that he had found that (although not as dramatic) the weakness could be targeted on the macOS version of Safari:

“With the current attack (CSS/HTML only), it will just freeze Safari for a minute then slow it down. You will be able to close the tab afterward.”

“To make it work on macOS, it requires a modified version containing Javascript. The reason why I did not publish it is that it seems that Safari persists after a forced reboot and the browser is launched again, therefore bricking the user’s session as the malicious page is executed once again.”

And if WebKit itself is vulnerable then it’s likely that there are many apps besides Safari that are at risk if they user WebKit for rendering webpages.

Haddouche has informed Apple about the vulnerability, which is believed to be investigating.

For now, without a patch available, there’s not much that users can do to prevent themselves from becoming the unwitting victims of the attack.

As always, be suspicious of links sent to you in unsolicited emails, and at least feel some consolation that this particular vulnerability is not going to lead to your private data being stolen.