A new study reveals that cryptocurrency miners are turning to porn sites to exploit users’ computing power when they’re online. A Bejing-based company called 360NetLab has analyzed internet traffic patterns and concluded that sites offering adult content, while small in terms of web traffic, make up the vast majority of sites containing web mining code on their homepages.

These cryptocurrency mining programs usually download without permission when you click on the site and run in the background, using the computing power of your CPU to create new cryptocurrency (usually Monero). This can significantly diminish the performance of your computer while you’re online.

According to 360NetLab, 628 websites out of Alexa’s top 300,000 contained cryptomining code within their homepage (Alexa is a web traffic and analysis service that ranks websites by visitors, not to be confused with the ubiquitous voice assistant of the same name).

While small in terms of actual traffic, the research found that a whopping 49 percent of the domains containing cryptomining software were sites offering adult content.

We’re not linking to any of these sites for obvious reasons, but in terms of traffic the most-visited sites were mejortorrent, firefoxchina, scamadviser, and thepiratebay.blue. The most prevalent coin-mining tool is coinhive, which is used by 57 percent of the sites. Others include jsecoin, webmine, and cryptoloot.

More and more mining site providers want a piece of the action as well. The coin-mining software downloads started to become more prevalent around September of last year, and saw a big surge in January.

As the International Business Times points out, what these companies are doing is not technically illegal in most countries. Some sites claim that cryptocoin mining is a way to fund their websites, just like hosting advertising or asking for subscriptions. Most users would argue that it should be opt-in only. In fact, one new site called authedmine only mines with the user’s permission.

600 websites is a pretty small number, however, so there’s no need to panic. Any decent antivirus software can protect your computer by detecting the mining software before it’s downloaded. As always, make sure your operating system and security software are up to date.