The popularity of CryptoCurrency has rapidly increased over the last 2 years and has exploded in the last 6 months. Mining has become more difficult and now requires data centers of dedicated servers to turn a healthy profit. A new mining technique utilizes the combined computing power of thousands of endpoints that mine through a script run in the browser, which can also lead to abuse.
Unit 42 found a few interesting facts and trends:
Websites out to monetize on their visitors while still providing their users with content. Several porn sites were seen to operate in this fashion.
A large percentage of the scripts detected in the PAN-DB feed appear to be hosted on a URL on the .bid or .download TLD
While the use of these mining scripts is not malicious in itself, no sites appered to notify the user their resources were being used in this manner, nor was an opt-out made available.
Enabling an adblock browser plugin like Adguard or Adblock plus will further assist in preventing these scripts from consuming resources.
Unit 42 has more statistics and a more thorough description of the scrips available through their article Unauthorized Coin Mining in the Browser and they also have a list of the top-ranking URLs hosting these scripts available here.