From information floating around on Twitter it looks like they had the password to the SSO account of an employee and then social engineered their way to get the employee to accept the push MFA prompt to add a new device.
At this point it appears that they found more credentials on the internal network and owned SSO, MFA and AD giving admin access to everything.
> found more credentials on the internal network ... giving admin access to everything
That's my hangup. The fact that admin/root level accounts can be accessed with "credentials" alone, rather than only via SSO/MFA/Yubikey. Were these service accounts, what happened to least privilege?
It depends on the employee you target. If it is someone working on internal IT systems, chances are high that you gain pretty wide access after owning their SSO.
SSO can go down or get owned so having break glass credentials isn't unheard of. The last place I worked at had them on paper in a safe in their headquarters. The Twitter threads show that they were stored in a password manager but the hacker was able to find credentials to access it which could have been one of the responsiblities of the employee which was targeted.
If you have your password manager on SSO it will be even easier.
At this point it appears that they found more credentials on the internal network and owned SSO, MFA and AD giving admin access to everything.