The owners of the Flashback botnet may have earned $14,000 (£8,800) in three weeks due to adverts being placed on compromised computers.
According to research by Symantec, the owners of Flashback displayed ads on more than ten million compromised computers for a three-week period in April, in which time around 400,000 clicks were made, earning a potential total of $14,000.
It also estimated that the ad-clicking component of Flashback was installed on only 10,000 of the 600,000-plus infected machines, utilising less than two per cent of the entire botnet.
Analysing the traffic delivered from the Flashback command-and-control servers, Symantec was able to follow the redirects used by the attackers. It found that the compromised computers were passing on users' search keywords, with the attackers then contacting various pay-per-click (PPC) services and routing the ads from these providers to the compromised computers – and earning money for doing so.
“We were able to identify patterns in the traffic sent to the compromised computers showing that the Flashback operators prefer some PPC providers over others. In fact, over 98 per cent of the ads being sent to compromised computers appear to originate from the same PPC provider,” it said.
“In such cases, the attackers are taking advantage of both users and the PPC providers by getting paid for ads that may not have been seen by users and may not be relevant to what the user searched for.
“Although PPC botnets are not a new idea (we have seen them on Windows for years), as the market share of Mac increases, we will see more Mac-related botnets similar to this one.”