Thousands of Microsoft Customers May Have Been Victims of Hack Tied to China

Businesses and government agencies in the United States that use a Microsoft email service have been compromised in an aggressive hacking campaign that was probably sponsored by the Chinese government, Microsoft said.

The number of victims is estimated to be in the tens of thousands and could rise, some security experts believe, as the investigation into the breach continues. The hackers had stealthily attacked several targets in January, according to Volexity, the cybersecurity firm that discovered the hack, but escalated their efforts in recent weeks as Microsoft moved to repair...