A specially crafted webpage will knacker Apple's Messages and Safari software on iOS and macOS, allowing miscreants to spread merry mischief by texting fans a link to the dastardly HTML.
The page also causes other programs, such as TextEdit on Macs, to hang when opened.
This is due to, from what we can tell, it being stuffed with characters that confuse the operating system's font-rendering code, resulting in applications hanging forever or being automatically killed.
The programming blunder in the iGiant's display code is triggered by, according to a note in the aforementioned webpage, overloading "the title property with massive characters containing heavy ligature."
This, it seems, causes the rendering routines, or components related to them, to enter an infinite loop and thus become unresponsive.
This leaves folks with non-working software.