There’re new security rules applied in Flash Builder 4. Everything works fine when you debug your application from localhost. However if you try to access your page from remote host you’ll come across an error: “Security error accessing url”. To resolve the situation you’ll have to create a file named: crossdomain.xml at the root of your domain. Sample crossdomain.xml can be found here.
That’s not all. If you want to access the site from remote hosts, just export the project from FlashBuilder! (Project->Export release build). In debug releases Flash forces browser to download crossdomain.xml from “localhost”. In release builds crossdomain.xml will be downloaded from the server.
<!DOCTYPE cross-domain-policy SYSTEM "/xml/dtds/cross-domain-policy.dtd">
<allow-access-from domain="*" to-ports="*" />
<allow-http-request-headers-from domain="*" headers="*"/>