A researcher has documented almost 2,500 sites that are actively running cryptocurrency mining code (Javascript e WebAssembly) in the browsers of unsuspecting visitors.
You can notice it because when someone is visiting one of these sites that embeds cryptocurrency mining code it maxes out the visitor’s CPU.
You can make a test by visiting the Cryptojacking test site to see if you are vulnerable.
How to block cryptojacking?
Some developers have realized browser extensions that can block these scripts:
NoCoin for Chrome, Firefox and Opera
NoMiner
Mining Blocker
minerBlock for Chrome
Coin-Hive Blocker for Chrome
Users that already have an Ad blocker can add the list from adblock-nocoin-list Github project to prevent mining scripts.