Adverts on Yahoo's homepage were infected with malware designed to mine the Bitcoin virtual currency, according to security experts.
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.
"The malware writers put a lot of effort into making it as efficient as possible to utilise the computing power in the best way," Light Cyber's founder Giora Engel told the BBC.
Lucrative market 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.
Other than a computer running slower, victims will be unaware that their machine is being used in what could become known as a "bitnet".
It is a variation on the traditional botnet, networks of malware-infected computers used to churn out spam or bombard websites with requests in order to knock them offline.
Some experts estimate that such networks could be generating as much as $100,000 (£60,000) each day.
Since bitcoins have risen in value - one bitcoin is now worth £1,000 - it is becoming a lucrative market for online criminals.
"Bitcoin mining malware is the new frontier as criminal gangs look for new ways to make money," said Mr Engel.
Easy target Yahoo acknowledged the attack in a statement earlier this week.
"From December 31 to January 3 on our European sites, we served some advertisements that did not meet our editorial guidelines - specifically, they spread malware," the statement read.
It went on to say that users in America, Asia and Latin America weren't affected but did not specify how many European users were victims.
Fox IT, the Dutch cybersecurity firm which revealed the malware attack, estimates that there were around 27,000 infections every hour the malware was live on the site.
Over the period of the attack that could mean as many as two million machines were infected.
Such attacks may be hard to avoid, said Mr Shulman.
"For an ad platform it is virtually impossible to guarantee 100% malware free ads."
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