Robert Silvie returned to his parents’ home for a Mardi Gras visit this year and immediately noticed something strange: common websites like those belonging to Apple, Walmart, Target, Bing, and eBay were displaying unusual ads. Silvie knew that Bing, for instance, didn’t run commodity banner ads along the bottom of its pristine home page—and yet, there they were. Somewhere between Silvie’s computer and the Bing servers, something was injecting ads into the data passing through the tubes. Were his parents suffering from some kind of ad-serving malware infection? And if so, what else might the malware be watching—or stealing?