(Phys.org) —Yahoo, has acknowledged that its service sites were used by hackers to enslave massive numbers of ordinary PCs who did so to generate bitcoins, and by extension, real earnings. Ads were placed on Yahoo web sites that when clicked by users, resulted in malware being installed on their computer. The malware reportedly caused no harm to user PC's other than to make them appear to run slower.
Bitcoin is a peer-to-peer payment system—created as a means to allow for commerce on the Internet without the assistance of a governmentally backed monetary system. Bitcoins can be used to purchase things from vendors (who accept them) on the Internet in ways nearly identical to using a credit card or PayPal. In order to do so, users must first obtain one or more them—most do so by either purchasing them directly or by earning them via offers made by others that have some to share. In order for the system to grow, more bitcoins must be generated on a regular basis. But they're not printed by a central agency, bitcoins are instead created by "miners" which are computers running software designed to solve math problems—they get a certain number of bitcoins in exchange for their efforts. As the popularity of bitcoins has increased, the math problems have become harder (limiting supply to keep up demand and thus value) requiring more processor power. In the beginning, a person using their own computer could use the software to create bitcoins, nowadays, it takes powerful dedicated computers or in this new effort, a botnet—which is where PCs were pooled together to solve the math problems.
With the Yahoo incident, a hacker group developed malware that infected user computers when their owner clicked on a certain ad, causing their computer to become part of a botnet (each doing a little bit of number crunching.) That botnet worked as a single entity (miner) to generate massive numbers of bitcoins, which all went to the originator of the malware.
Yahoo says only Yahoo users in Europe were infected by the malware and only for the period December 31, 2013 through January 3, 2014—when Yahoo discovered the ruse it removed the ads. Unfortunately, all of those computers (approximately two million) infected during that timeframe remain infected unless their owner has taken steps to have the malware removed using commercial anti-virus software.