Iframe problem

I’m not quite sure how you’d build a shared login system using iframes. There are technical restrictions on how much data you can share between the parent page and the child page for security reasons, enforced by all browsers. OAuth/OIDC would be the most elegant way to solve this, which is the same protocol that’s also used for Google logins, but that may be difficult because it also involves API interactions between websites, which is also blocked on our hosting. One good thing to be aware of: multiple websites can talk to the same database. So you could either use that to have a single user database shared between multiple websites (which I think is how @Jri-Creator does it), or use the database connection to replace the API layer for the OAuth login flow.