The United States and international partners are blaming China with “high confidence” for the cyber campaign exploiting zero-day vulnerabilities in Microsoft Exchange reported by the company in March and announced new joint measures to “strengthen our collective cyber resilience and security cooperation,” a senior administration official told reporters late Sunday.

“We’ve raised our concerns about both the Microsoft incident and the PRC’s broader malicious cyber activity with senior PRC government officials, making clear that the PRC’s actions threaten security, confidence, and stability in cyberspace,” the official said. “The U.S. and our allies and partners are not ruling out further actions to hold the PRC accountable.”

The official said that the investigation behind the attribution “really gave us new insights on the MSS’s work and on the kind of aggressive behavior that we’re seeing coming out of China.” The “breadth of compromises, tens of thousands of systems around the world” also delayed attribution for the Microsoft Exchange hack until now.

Read more: HS Today