Every so often, a new piece of malicious software comes along that introduces a subtle yet evolutionary technological leap, a quickly-mimicked shift that allows cyber crooks to be far more stealthy in plying their trade. According to research released last week, this happened most recently in the realm of click fraud, a rapidly growing problem that inflates online advertising costs for legitimate companies and ad networks. For years, hackers have used malicious software to perpetrate click fraud by hijacking the results displayed when users search for something online. The trouble is, these scams can be rather clumsy: Victims often figure out pretty quickly that something is wrong, usually because their searches are redirected to an unfamiliar search portal, as opposed to their regular default search provider. But a new Trojan horse program being distributed by tens of thousands of recently hacked Web sites hijacks search results so that Google.com users