Scenario then vs now:
Websites using crypto-miner services could mine cryptocurrencies with your browser memory when you visit their site.
Feasibility: Once you close the browser window, they lost access to your processor and associated resources, which eventually stops mining.
Currently, some websites have put a newer technique a clever trick to keep their cryptocurrency mining software running in the background even when you have closed the browser!
How does the new technique work?
1. It opens a hidden pop-under browser window that fits behind the taskbar and hides behind the clock on your Microsoft’s Windows computer.
2. The website runs the crypto-miner code that indefinitely generates cryptocurrency for the person controlling the site while eating up CPU cycles and power from your computer until and unless you close the window.
3. It works on the latest version of Google’s Chrome web browser running on the most recent versions of Microsoft’s Windows 7 and Windows 10.
Block Hidden Cryptocurrency Miners:
1. Look for any browser windows in the taskbar.
2. Kill it.
3. Use web browser extensions, like No Coin, that automatically block in-browser cryptocurrency miners for you, and regularly update themselves with new mining scripts that come out.
More at: https://blog.malwarebytes.com/cybercrime/2017/11/persistent-drive-by-cryptomining-coming-to-a-browser-near-you/