Multiple hacking groups—including state actors from China—have targeted a vulnerability in older, on-premises versions of the file-sharing tool after a flawed attempt to patch it.

Microsoft issued an alert on Saturday that said the server software being targeted is used by government agencies and businesses to share documents within their organizations.

The US agency overseeing nuclear weapons breached in a China-linked hack of Microsoft SharePoint, according to Bloomberg. Officials say no classified data was exposed and the impact was limited.