alt Hacker Newsfailed to properly validate the originating tenant
One wonders whether those who designed all this ever considered what that field in the token is for.
The word "tenant" is also very telling --- you're just renting, and the "landlord" always has the keys.
It's even worse: "Because of the nature of these Actor tokens, they are not subject to security policies like Conditional Access". This goes against all principles of good security design. A token that gives root access instead of specifying a particular action allowed just invites misuse, erroneous or malicious.
I would expect these tokens to be like JWT or macaroons, carrying specific permissions within specific bounds / tenants. Alas.