As many as two million European users of Yahoo may have received PC malware from virus-laden ads served by its homepage over a four-day period last week.
Some of the malware would turn PCs into bitcoin miners – a huge drain on its computing resources – without users’ knowledge. Yahoo has been criticised for not saying how many people could be affected or doing anything to help those with the malware, which attacked flaws in Java modules on systems.
In a statement, Yahoo said: “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.” Users in North America, Asia Pacific and Latin America weren’t affected, Yahoo said. Nor were users of Apple Macs or mobile devices.
“We will continue to monitor and block any advertisements being used for this activity,” the company added. “We will post more information for our users shortly.”
According to Light Cyber, a security research firm which warned Yahoo of the attacks in late December, one of the malware programs delivered in the attack turned the victim’s computer into a bitcoin miner. The computer is set to work performing the calculations required to make the bitcoin network run, but the rewards for doing so accrue to the malware writer.
Yahoo has been criticised for not doing more to aid users infected by the faulty adverts. Dan Farber of technology site CNET says that: “At this point, Yahoo hasn’t addressed any of the details, such as how the malware exploit got into its Web pages, how many users are impacted, and what victims of the attack should do. The company may still be gathering data.”