Many longtime internet users will remember receiving pop-up ads warning that their computers were infected with a virus. In nearly all cases, the ad’s specific claims were bogus; the purpose was to scare users into paying for a questionable tech support service or to drive them to a site that would actually infect them with malware.

While browser-based pop-up blockers have largely killed off that particular scam, malicious advertising — or malvertising — is still causing serious damage. Purveyors of malvertisements use an increasingly broad range of techniques to insert malware into ads that run across the web on large advertising networks.