Microsoft blamed two Chinese nation-state actors for exploiting recently discovered security flaws in SharePoint to infiltrate vulnerable organizations, like schools, state governments, and the U.S. government’s top nuclear security agency.

Tech giant Microsoft has said Chinese hacking groups were believed to be behind the recent attacks on its SharePoint collaboration software.

The company said state-backed hacking groups were breaching systems through flaws in SharePoint, which is used by the U.S. government and companies around the world.