Microsoft said in a post on its website on Saturday that it was “aware of active attacks targeting on-premises SharePoint Server customers by exploiting vulnerabilities.” SharePoint is a Microsoft platform that allows customers to manage and share documents within their organizations.

An investigation is underway after hackers used a security flaw in Microsoft software to internationally infiltrate agencies and businesses over the weekend.

A warning has been issued to Microsoft users detailing a cybersecurity flaw that allowed hackers to access its SharePoint servers, the U.S. Cybersecurity and Infrastructure Security Agency (CISA) announced.