Using cross-domain policy files
To make data available to SWF files in different domains, use a cross-domain policy file. A cross-domain policy file is an XML file that provides a way for the server to indicate that its data and documents are available to SWF files served from other domains. Any SWF file that is served from a domain that the server's policy file specifies is permitted to access data or assets from that server.
When a Flash document attempts to access data from another domain, Flash Player attempts to load a policy file from that domain. If the domain of the Flash document that is attempting to access the data is included in the policy file, the data is automatically accessible.
Mythical Man Month