Yahoo confirmed that for a four-day period in January, malware was served in ads on its homepage.
Experts estimate that as many as two million European users could have been hit.
Security firm Light Cyber said the malware was intended to create a huge network of Bitcoin mining machines.
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Bitcoin mining malware is designed to steal computing power to make it easier for criminals to accumulate the virtual currency with little effort on their part.
"Generating bitcoins is basically guessing numbers," said Amichai Shulman, chief technology office of security firm Imperva.
"The first one to guess the right number gets 25 bitcoins and if you have a large volume of computers guessing in a co-ordinated way then you have a more efficient way of making money," he added.The BBC reports that using malware to take control of unsuspecting computers allows criminals to make a lot of money, and cites an expert who labels the tactic of using botnets to harvest bitcoins as a "new frontier" for criminal gangs.