I use object. But the app using the iframe would need permission to use it. No different than an API, how do you know what user at a company is getting the API data.
That is where their local security comes it, example at a Humane Society an admin could add and edit animals, so they needed admin role to go to the page that had the API login for Adopt a pet.
Note: Local policy and the API policies are totally two different things. You still have API policies.
Example Humane society X cannot edit Humane society Z data.
It can get complex, so write all roles and policies out with pencil and paper and make them make sense.
This part of an App is the most complex, not an hour thing, but days.