Cyber-security experts are warning that criminals are hacking into other people's machines to mine cryptocurrencies for them.

According to a Trend Micro report, school, charity and file-sharing websites have been found infected with a particular code that makes the visitor's machine mine cryptocurrency.

That code turns the machine into a "miner." By creating a huge network of machines, hackers can quickly generate huge amounts of money.

"This is absolutely a numbers game," said Rik Ferguson, vice-president of security research at Trend Micro. "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 million of the world's most popular sites have been scanned, and many are using the JSE Coin script. Some do it on purpose, to earn a bit extra off their visitors (for roughly a million visits a site would get around £88 in Monero cryptocurrency). Others, however, have no idea how the code ended up on their site.

Some have already removed it, updated their security policies and are currently investigating how the code was implanted in the first place.

Even the creators of Coin Hive, one of the scripts used for this purpose, said they're taking action against malicious use. "We had a few early users that implemented the script on sites they previously hacked, without the site owner's knowledge," they said in a message to the BBC. "We have banned several of these accounts and will continue to do so when we learn about such cases."

Cryptocurrency is gaining popularity and value. Bitcoin's current value is around $4,800, while Ethereum, the second largest coin, sits at $300.

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