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According to a recent report from AdGuard, more than 200 of the top 100,000 sites on the web were hosting suspicious code. That may not seem like a big deal, but even sites near the bottom of that list draw a huge amount of traffic. The most popular one on AdGuard's list regularly pulls in around 60 million visits every month.

In total, AdGuard discovered cryptocurrency mining code on 220 sites over a three-week period. During that time around 500 million users unknowingly ran that code on their computers.

More than half a billion people may be inadvertently mining cryptocurrencies from their computers, smartphones and other devices, according to research conducted earlier this year by ad blocking firm AdGuard.

“How much money have these websites made? We estimate their joint profit at over $43,000,” the AdGuard researchers said in an October blogpost that detailed their discovery. At the time, one bitcoin was worth around $5,000.

The video sites Openload, Streamango, Rapidvideo and OnlineVideoConverter are allegedly loading mining software on to visitors’ computers, making them generate tokens for the bitcoin-like cryptocurrency Monero, according to security firm Adguard.

“We came across several very popular websites that secretly use the resources of users’ devices for cryptocurrency mining,” said Andrey Meshkov, co-founder of Adguard. “According to SimilarWeb, these four sites register 992m visits monthly.”

Although the developers aren't saying how much money they've made from their idea, online ad-blocking service AdGuard also found the Coinhive miner on over 30,000 sites and estimates the code generates $150,000 in Monero every month. For Coinhive, which takes a 30 percent cut, that amounts to $540,000 per year.

Popular video sites including Openload, Streamango, Rapidvideo and OnlineVideoConverter are also allegedly loading mining software on to visitors' computers, according to security firm Adguard.

"We came across several very popular websites that secretly use the resources of users’ devices for cryptocurrency mining," said Andrey Meshkov, co-founder of Adguard. "According to SimilarWeb, these four sites register 992m visits monthly."

AdGuard will flat-out refuse to connect to websites known to be malicious for phishing attacks or spreading malware, and it’s good at collapsing the elements of the page that normally display ads, making formatting much less cluttered.