Here is the report (pdf) – (archived)
Oasis Security’s research team uncovered a critical vulnerability in Microsoft’s Multi-Factor Authentication (MFA) implementation, allowing attackers to bypass it and gain unauthorized access to the user’s account, including Outlook emails, OneDrive files, Teams chats, Azure Cloud, and more. Microsoft has more than 400 million paid Office 365 seats, making the consequences of this vulnerability far-reaching.
The bypass was simple: it took around an hour to execute, required no user interaction and did not generate any notification or provide the account holder with any indication of trouble.
[Edit to insert the original link to the Oasis site.]
No, it’s a MFA bypass. All a hacker needed was the ability to initiate new sessions (after stealing user:pass, for example via malware).