iPhone hacking techniques have sometimes been described almost like rare and elusive animals: Hackers have used them so stealthily and carefully against such a small number of hand-picked targets that they're only rarely seen in the wild. Now a recent spate of espionage and cybercriminal campaigns has instead deployed those same phone-takeover tools, embedded in infected websites, to indiscriminately hack phones by the thousands. And one new technique in particular—capable of taking over any of hundreds of millions of iOS devices—has appeared on the web in an easily reusable form, putting a significant fraction of the world's iPhone users at risk.

Researchers at Google and cybersecurity firms iVerify and Lookout on Wednesday jointly revealed the discovery of a sophisticated iPhone hacking technique known as DarkSword that they've seen in use on infected websites, capable of instantly and silently hacking iOS devices that visit those sites.…