Hundreds of thousands of Yahoo users in European countries including the UK, France and Romania may have been infected with malware injected into advertising hosted on Yahoo websites.
Dutch company Fox-IT made the discovery at the end of last week, saying users who visited Yahoo would be confronted with advertisements that would direct them to external sites hosting malware that exploits Java security flaws.
The ads were served in iframes by Yahoo's advertising service, and were hosted on external sites. Upon clicking the advertisements, users would be redirected to a selection of other domains, all reporting the same Netherlands-based IP address.
The malware users were faced with include money-grabbing Zeus Trojan, botnet software Andromeda and other malware associated with advertising.
Fox-IT estimates that the traffic to the malware hosts was around 300,000 visits per hour. On the assumption that nine percent of those users would ultimately be infected, the firm says around 27,000 infections could have occurred every hour over the four days the adverts were present.
"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," Yahoo confirmed in a statement. "On January 3, we removed these advertisements from our European sites."
The statement added that users in other regions and those using Apple's Mac OS and mobile devices were not affected.
"We will continue to monitor and block any advertisements being used for this activity. We will post more information for our users shortly," Yahoo said.
The attack bore a resemblance to the one carried out on the PHP.net website, Fox-IT said. In recent months, popular services including Dropbox and Microsoft Silverlight have found themselves under attack, highlighting the risk businesses face when allowing employees to use personal service on their work devices, or allowing them to bring personal devices into work.