JupyterHub has a privilege escalation vulnerability with the `admin:users` scope

Summary

If a user is granted the admin:users scope, they may escalate their own privileges by making themselves a full admin user.

Details

The admin:users scope allows a user to edit user records:

> admin:users
>
> Read, write, create and delete users and their authentication state, not including their servers or tokens.
>
> – https://jupyterhub.readthedocs.io/en/stable/rbac/scopes.html#available-scopes

However, this includes making users admins. Admin users are granted scopes beyond admin:users making this a mechanism by which granted scopes may be escalated.

Impact

The impact is relatively small in that admin:users is already an extremely privileged scope only granted to trusted users.
In effect, admin:users is equivalent to admin=True, which is not intended.

Note that the change here only prevents escalation to the built-in JupyterHub admin role that has unrestricted permissions. It does not prevent users with e.g. groups permissions from granting themselves or other users permissions via group membership, which is intentional.