A cybersecurity firm says it has uncovered a massive online scam that employs an army of automated web browsers to siphon millions of dollars of advertising per day away from US media companies and brand-name advertisers. The firm, New York-based White Ops, has dubbed the operation “Methbot,” because of references to meth buried in the computer code underlying the scam, and says it is the largest and most profitable fraud operation yet to strike digital advertising.