Starting today, Google’s Chrome Web Store will no longer accept extensions that mine cryptocurrency. The company says there has been a rise in “malicious extensions” over the past few months which appear to be useful but are actually embedding hidden cryptocurrency mining scripts that run in the background without consent, consuming valuable computing resources. Google says existing extensions will be delisted from the Chrome Web Store in late June, though extensions with blockchain-related purposes “other than mining” will still be permitted.

Google had previously allowed cryptocurrency mining in extensions as long as it was the extension’s single purpose and the user was informed about the process. However, Google said that around 90 percent of all extensions with mining scripts submitted by developers have failed to comply with Google’s policies and have been rejected or removed from the store as a result.

Mining can negatively impact system performance as it takes up significant CPU power to run the rigorous calculations needed to mine the digital tokens. Companies and hackers have also been known to plant backdoor coin mining scripts in public websites to appropriate computational power from unsuspecting browsers.

In a blog post, Google’s extensions platform product manager James Wagner said:

“The extensions platform provides powerful capabilities that have enabled our developer community to build a vibrant catalog of extensions that help users get the most out of Chrome. Unfortunately, these same capabilities have attracted malicious software developers who attempt to abuse the platform at the expense of users.”

Cryptocurrencies are notoriously volatile and regulators as well as tech companies have increasingly sought to clamp down on nefarious activity. Google, Facebook, and Twitter all recently banned ads related to the digital tokens.