Many of these rogue ads appear as sponsored content during a search engine query on a desktop or mobile device. But malicious code can also be hidden in ads that appear on mainstream websites consumers routinely visit. Some of these ads will only ensnare consumers who click on them, but in some cases, people can be vulnerable in a more passive way — sometimes just by visiting an infected site, said Erich Kron, security awareness advocate for KnowBe4, a security awareness and training company.