The blog provided the most fulsome explanation yet for a hack that rattled both the cybersecurity industry and China-U.S. relations. Beijing has denied any involvement in the spying.
Microsoft and U.S. officials said on Wednesday night that Chinese state-linked hackers had been secretly since May accessing email accounts at around 25 organizations. U.S. officials said those included at least two government agencies: the State and Commerce Departments.
Secretary of State Antony Blinken told China's top diplomat, Wang Yi, in a meeting in Jakarta on Thursday that any action that targets the U.S. government, U.S. companies or American citizens "is of deep concern to us, and that we will take appropriate action to hold those responsible accountable," according to a senior State Department official.
Microsoft's blog post did not explain how the hackers got their hands on one of the company's digital keys, leading some experts to speculate that Microsoft itself had been hacked ahead of the thefts.
The company did not immediately respond to questions about the key.
The breach has thrown Microsoft's security practices under scrutiny, with officials and lawmakers calling on the Redmond, Washington-based company to make its top level of digital auditing, also called logging, available to all its customers free of charge.
Microsoft said in a statement late on Thursday that it was taking the criticism on board.
"We are evaluating feedback and are open to other models," the company said, adding that it was "actively engaged" with U.S. officials on the matter.
