As expected, it is money.
According to Symantec researchers, the malware's ad-clicking component is loaded into Chrome, Firefox, and Safari where it can intercept all GET and POST requests from the browser.
It then targets search queries made on Google and may redirect users to another page of the attacker's choosing. Consequently, the attackers receive payment for the ad click instead of Google.
"The ad click component parses out requests resulting from an ad click on Google Search and determines if it is on a whitelist. If not, it forwards the request to the malicious server," they explain.
This request is specially crafted in order to thwart researchers' attempts to investigate the URL.
"Ad-clicking Trojans are nothing new and in an analysis of W32.Xpaj.B last August a botnet measuring in the region of 25,000 infections could generate the author up to $450 per day," point out the researchers.
"Considering the Flashback Trojan measures in the hundreds of thousands, this figure could sharply rise to the order of $10000 per day," they concluded.