Half a billion people are making monthly visits to websites that may be secretly hijacking the processing power of their computers to mine cryptocurrency, security researchers recently warned.

Experts at AdGuard reported that 220 of the world’s most popular websites are running cryptocurrency mining scripts. Many of these sites contain torrents, videos or adult content, which keep users engaged for longer periods of time.

The Dangers of Cryptocurrency Mining Without Consent

When users visit these sites, scripts such as JSEcoin and CoinHive work in their browsers to verify blockchain transactions, which tend to require significant computational resources. In the last three weeks, AdGuard estimated that the fraudsters behind these schemes have made approximately $43,000.

ZDNet suggested that the sites discovered by AdGuard are merely following the lead of torrent site Pirate Bay, which admitted to mining Monero cryptocurrency without visitors’ consent. Though ostensibly less dangerous than infecting a device with malware, this kind of activity could cause the central processing unit (CPU) of a machine to wear out quickly and should not be done without users’ knowledge.

While the sites mining cryptocurrency may not need to run online ads that annoy their visitors, there should still be some way to opt out or turn these scripts off. According to Bitcoin, the makers of Coinhive publicly asked those using its script to adopt a permission-based model, but it may be impossible to make such requests compulsory.

Rising Value of Cryptocurrencies Driving Increased Attacks

In some cases, website operators might be just as surprised as their visitors. TechCrunch reported that PolitiFact, a fact-checking website, was running a script to mine cryptocurrency but has since removed it. The site is now conducting an investigation into the script’s origins.

Consumers can install ad blockers or use antivirus software features to prevent or remove in-browser cryptocurrency mining, but occasionally they may need to recognize it as the source of performance slowdowns. The rising values of bitcoin and Monero may make this kind of breach much more common — not to mention more lucrative.