This is the best summary I could come up with:

Such keys, Microsoft said, are entrusted only to employees who have undergone a background check and then only when they are using dedicated workstations protected by multi-factor authentication using hardware token devices.

To safeguard this dedicated environment, email, conferencing, web research, and other collaboration tools aren’t allowed because they provide the most common vectors for successful malware and phishing attacks.

The hack of a Microsoft engineer’s corporate account allowed Storm-0558 to access the crash dump and, with it, the expired Exchange signing key.

Addressing the second mystery, the post explained how an expired signing key for a consumer account was used to forge tokens for sensitive enterprise offerings.

Human errors prevented a programming interface designed to cryptographically validate which environment a key from working properly.

Thus, the mail system would accept a request for enterprise email using a security token signed with the consumer key (this issue has been corrected using the updated libraries).

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