http://www.techradar.com/news/your-computer-could-be-being-used-to-mine-for-cryptocurrency

Your computer may be harvesting cryptocurrencies without your permission thanks to a large number of popular websites reportedly being infected with malware that uses visitor’s computers to mine cryptocurrencies. 

According to a BBC report, a scan of some of the most popular websites on the web found hundreds to be harbouring malicious mining code.