Microsoft mitigates China-based threat actor Storm-0558 targeting of customer email

UPDATE: Microsoft performed a comprehensive technical investigation into the acquisition of the Microsoft account consumer signing key, including how it was used to access enterprise email. Our technical investigation has concluded, and on September 6, 2023, we published our investigation findings.
Microsoft has released threat analysis on Storm-0558 activity here. Microsoft additionally released additional defense-in-depth security fixes to help customers improve token validation in their custom applications.