School, charity and file-sharing websites have been caught out by scammers who are using them to generate crypto-cash, BBC reported.
Hackers have managed to install code on the sites that uses visitors' computers to "mine" the cyber-currencies.
Rik Ferguson, vice president of security research firm Trend Micro, said hackers use the malicious code to target as many computers as possible to establish a cryptocurrency mining network.
"There's a huge attraction of being able to use other people's devices in a massively distributed fashion because you then effectively take advantage of a huge amount of computing resources," he said.
A scan of the code behind millions of most popular websites has revealed that many are running the widely used Coin Hive mining script and others, such as JSE Coin, legitimately to generate some money from their steady stream of visitors.
On many sites found in the scan, the way the script was concealed suggested it had been uploaded surreptitiously.
Metrics published on the Coin Hive site suggest that a site that gets one million visitors a month would make about $116 in the Monero (XMR/USD) crypto-currency by mining.