YouTube was recently caught displaying ads that covertly leach off visitors' CPUs and electricity to generate digital currency on behalf of anonymous attackers, it was widely reported. You can read about it here:
YouTube Is Serving Covert Cryptocurrency Mining Ads (https://www.extremetech.com/internet/262934-youtube-serving-covert-cryptocurrency-mining-ads)
Malvertising Campaign Abuses Google's DoubleClick to Deliver Cryptocurrency Miners (https://blog.trendmicro.com/trendlabs-security-intelligence/malvertising-campaign-abuses-googles-doubleclick-to-deliver-cryptocurrency-miners/)
Now even YouTube serves ads with CPU-draining cryptocurrency miners (https://arstechnica.com/information-technology/2018/01/now-even-youtube-serves-ads-with-cpu-draining-cryptocurrency-miners/)