YouTube has been briefly caught out by adverts that sought to mine valuable crypto-coins using visitors' computers.
The malicious adverts were spotted by security firm Trend Micro, which watches crypto-cash malware.
The adverts included computer code that helped them mine the Monero crypto-currency.
The rapid rise in the value of crypto-coins has prompted many cyber-crime gangs to try and cash in by using innocent machines to generate the cash.
Cashing out
The gangs are keen to get malicious code on lots of computers because most crypto-currencies rely on large networks of machines to verify transactions and generate new coins. The more machines working for them, the more coins they can extract.
Chaoying Liu and Joseph Chen from Trend Micro said the first sign of the campaign was a sudden tripling of the number of malicious coin-mining scripts the company caught.