Would like to embed Confluence into an iframe on an external corporate site.

[Brad Lauzon]

Should be as simple as adding the https:// url for the Confluence page but get security errors that it is against policy. 

Any other site, internal or external, works when the url is plugged in.  Using REST API is not an option.

[Milo Test]

Might be a permissions problem. You probably need to add anonymous read access to the site and space.