The Origin check of the MCP transport, against DNS rebinding.

A request without an Origin header (every client that is not a browser) passes. A request with one must name a host this install knows, else it is answered 403 origin_not_allowed before any key is looked at (the MCP plug runs this as the :before_auth hook of PhoenixKit.API.Auth).

What is allowed

• the install's canonical host: the host of the site_url setting, else of the endpoint's URL (the same source as PhoenixKit.Utils.Routes.base_url/0, read through the settings cache because this check runs before any rate limit);
• when the host app configures config :phoenix_kit, :api_request_context, {Module, :function}, the :hosts that function explicitly returns (every domain of a multi-domain install). A hook that returns no :hosts key (a hook that sets only language and currency), an empty or non-string list, junk, or that raises or throws contributes nothing. The hook is called with the conn, and it runs again later in the same request when the request context is resolved, so it must be cheap and free of side effects;
• the entries of the api_mcp_allowed_origins setting, which are full origins: https://client.example matches scheme, host and port. An entry without :// never matches (the Settings page stores only full origins).

The request's own Host header is deliberately not on the list. A DNS-rebinding page sends Host: evil.test and Origin: http://evil.test; both come from the attacker, so a list built from the Host header would admit it. The default PhoenixKit.API.RequestContext host list is exactly that header, and RequestContext.resolve/1 keeps it whenever a hook returns no :hosts or fails, which is why this module does not use resolve/1 and calls the hook itself.

An origin with userinfo, a path, a query, a fragment, a scheme other than http or https, no host (null, file://), surrounding whitespace or bytes that are not valid UTF-8 is never allowed. A request with two Origin headers is refused.