why does a site loaded in an iframe not recognize that i am logged in to that site in another tab?

I am logged into a phpBB site in a Firefox tab.

On my site I load the same phpBB site in an ifra开发者_如何学编程me in another tab of Firefox.

When I load the iframe it takes me to the login page.

If I open the phpBB site again in a third tab, I am still logged in.

Is this because a new session ID is being issued because of the iframe? Or maybe it's because of the session cookies?

Thanks

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It might be to protect against click-jactking. If the page detects that it is in a tab, it loads the login page instead. Try reloading the page with JavaScript turned off.