One output of the most mainstream sites discovered hundreds harboring the malevolent mining code.
By getting bunches of PCs to join the systems, assailants can rapidly create money.
"This is totally a numbers amusement," said Rik Ferguson, VP of security look into at Pattern Miniaturized scale.
The calculating is called mining and new crypto-coins are passed out to diggers who are the first to illuminate the mind boggling totals.
The more PC control that somebody can hoard, said Mr Ferguson, the more coins they can produce.
"There's a gigantic fascination of having the capacity to utilize other individuals' gadgets in an enormously dispersed manner since you at that point viably exploit a tremendous measure of registering assets," he said.
A security researcher has scanned the code behind the million most popular websites to see which ones are running the widely used Coin Hive mining script.
Many sites use this and others, such as JSE Coin, legitimately to generate some money from their steady stream of visitors. Metrics published on the Coin Hive site suggest that a site that gets one million visitors a month would make about $116 (£88) in the Monero crypto-currency by mining.
On many sites found in the scan, the way the script was concealed suggested it had been uploaded surreptitiously.
The BBC contacted several of the sites in the UK running the Coin Hive script and those that responded said they did not know who added it to their site. Some have now deleted the mining code, updated their security policies and are investigating how the code was implanted.
Coin Hive's developers said it had also taken action against malicious use.
