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School, charity and file-sharing websites have been caught out by scammers who are using them to generate crypto-cash.
Hackers have managed to install code on the sites that uses visitors' computers to "mine" the cyber-currencies.
One scan of the most popular websites found hundreds harbouring the malicious mining code.
By getting lots of computers to join the networks, attackers can quickly generate cash.
"This is absolutely a numbers game," said Rik Ferguson, vice-president of security research at Trend Micro.
Mr Ferguson said crypto-currencies operated by getting lots of computers to work together to solve the tricky mathematical problems that establish who spent what. This establishes a digital ledger, or blockchain, of spending activity with a particular coin.
The number crunching is called mining and new crypto-coins are handed out to miners who are the first to solve the complex sums.
The more computer power that someone can amass, said Mr Ferguson, the more coins they can generate.
"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.