Advertising can sometimes be annoying — and sometimes it can be malicious. Businesses that make their money selling advertisements sometimes go too far trying to make sure you see their ads. Recently researchers found that one such business — a big digital-marketing agency — went as far as installing adware on 250 million computers running Windows and macOS all over the world.

What’s even worse, this adware is capable of turning into full-fledged malware that can divert users to malicious sites and drop malware on their computers. And no one seemed to notice it — until now.