Apparently this had been going on for a couple of years...
"The criminals were able to send banner ads and javascript to their targets' computers by pushing both into ad networks. These networks aggressively scan advertisers' javascript for suspicious code, so the criminals needed to sneak their bad code past these checks. To do this, they made tiny alterations to the transparency values of the individual pixels of the accompanying banner ads, which were in the PNG format, which allows for pixel-level gradations in transparency. The javascript sent by the attackers would run through the pixels in the banners, looking for ones with the telltale alterations, then it would turn that tweaked transparency value into a character. By stringing all these characters together, the javascript would assemble a new program, which it would then execute on the target's computer. This new program triggered a network request to a site controlled by the attackers, which repeatedly checked the target's computer to see if it was running inside a virtual machine (a telltale sign of a paranoid user, possibly a security researcher who would figure out what was going on) or whether it had any anti-virus software. Once it was satisfied that the target was not in a position to detect active attacks, it launched exploits targeted at Internet Explorer/Flash to hijack the machine and gather the user's keystrokes, with a special emphasis on bank-industry information."
http://boingboing.net/2016/12/07/for-two-years-criminals-stole.html More: http://arstechnica.com/security/2016/12/millions-exposed-to-malvertising-tha...