What
Cryptojacking is the secret use of a computing device to mine the digital cash.
Who
Hackers use malicious softwares that focuses on stealing cryptocurrency wallets or mining, and Sites use in-browser JavaScript to mine in the background.
How
JavaScript runs on all website, so in-browser mining code start working instantly when their web-page is loaded. Even Hackers have injected the scripts into other websites; some affected sites are CBS Showtime, UFC live-streams and even some official government websites.
Why
Apat from criminals, now due to Adblockers, many legit websites have also started using this way for their revenue.
Protection:-
Block such sites;
Use proper browsers which checks Java Script,
Repeatedly check the running tasks in your device,
Never compromise with your internet-security, there is no fake alert about malicious softwares.