Swarms of hacked, internet-connected devices have menaced core parts of the web and put utilities on edge. On Friday, millions of Americans awoke to find they couldn't access many popular websites, including Twitter, Spotify and Github. Hackers had overwhelmed the cyberdefenses of Dyn, a New Hampshire-based company that manages web traffic for thousands of U.S. sites. What made the attack stand out was not only its size and scope but its source: a digital army of webcams, CCTV devices and other "smart" electronics that had been infected with the Mirai malware.