Dozens of plug-ins for the widely used open source web blogging software WordPress are now offline after a backdoor was discovered in them, used to push malicious code to any website that relied on the plug-ins. The backdoor was discovered after a new corporate owner bought these plug-ins.

Anchor Hosting founder Austin Ginder sounded the alarm in a blog post last week describing a supply chain attack on a WordPress plug-in maker called Essential Plugin. Ginder said someone last year bought Essential Plugin and the backdoor was soon added to the plug-ins’ source code. The backdoor sat dormant until earlier this month when it activated and began distributing malicious code to any website with the plug-ins installed.

Essential Plugin says on its website that it has over 400,000 plug-in installs and more than 15,000 customers. WordPress’ plug-in install page says the affected plug-ins are in over 20,000 active WordPress installations.

Plug-ins allow owners of WordPress-based websites to extend the site’s functionality, but in doing so grant the plug-ins access to their installations, which can open these websites to malicious extensions and potential compromise. But Ginder warned that WordPress users are not notified of any plug-ins’ change in ownership, exposing users to potential takeover attacks by their new owners.

According to Ginder, this is the second hijack of a WordPress plug-in discovered in as many weeks. Security researchers have long warned of the risks of malicious actors buying software and changing its code in order to compromise a large number of computers around the world.

While the plug-ins have been removed from WordPress’ directory and now list their closure as “permanent,” Ginder warned that WordPress owners should check if they still have one of the malicious plug-ins installed and remove it. Ginder has a list of the affected plug-ins in the blog post.

Representatives for Essential Plugin did not respond to a request for comment.