Apple iPhones, iPads, and Mac computers that stray onto websites with malicious CSS code, while using Safari, can crash or fall over – due to a flaw in the web browser.
The WebKit rendering engine vulnerability can be triggered by just a few lines of code in a cascading style sheet (CSS). On iOS devices, at least, it all starts to go wrong when the browser tries to parse a processor-intensive CSS feature called backdrop-filter on nested page elements.
The so-called Safari Reaper attack – developed by Berlin-based security researcher Sabri Haddouche and uploaded to GitHub this week – effectively crashes iOS devices, from iPads to iPhones running iOS 7 to 12, and even Apple smartwatches. The CSS causes the rendering engine to exhaust the system's resources, and force the gadgets to reboot to recover.
Macs can be similarly frozen by the same exploit, forcing them to restart, so don't try this at home. Other browsers that make use of WebKit are likely also vulnerable. On systems that don't crash, the HTML renders a picture of a "triggered" Thomas the Tank Engine.
Haddouche, who works for secure messaging outfit Wire, told El Reg that the same trick crashes tabs on IE and Edge. The researcher came across the vulnerability while researching browser-crashing attacks more generally last week.
He suggested restricting nested elements and expensive CSS calls to defend against the attack.
The method is reminiscent of the "evil text" iPhone bug of 2015 except in this case we're talking about CSS. Neither vulnerability can push malware onto crashed devices so it's more a nuisance than anything else, at least by itself – it's a way to crash your pal's iThing or Mac by tricking them into opening a dodgy page in Safari. ®
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