A cluster of more than 120,000 hijacked computers has been bombarding websites with fake traffic, costing online advertisers more than $6 million a month, according to research by Spider.io. The network, known as the Chameleon botnet, is thought to be responsible for around 9 billion bogus ad impressions a month across 202 websites. The botnet is said to be particularly sophisticated, mirroring human browsing, ad-clicking and mouse-tracking activities, with fake traffic only identifiable using sophisticated analytic techniques.