The browser security model normally prevents web content from one domain from accessing data from another domain. This is commonly known as the "same origin policy". URL policy files grant cross-domain permissions for reading data. They permit operations that are not permitted by default. The URL policy file is located, by default, in the root directory of the target server, with the name crossdomain.xml (for example, at www.example.com/crossdomain.xml).
When a domain is specified in crossdomain.xml file, the site declares that it is willing to allow the operators of any servers in that domain to obtain any document on the server where the policy file resides. The crossdomain.xml file deployed on this website opens the server to all domains (use of a single asterisk "*" as a pure wildcard is supported) like so:
This practice is suitable for public servers, but should not be used for sites located behind a firewall because it could permit access to protected areas. It should not be used for sites that require authentication in the form of passwords or cookies. Sites that use the common practice of authentication based on cookies to access private or user-specific data should be especially careful when using cross-domain policy files.
Carefully evaluate which sites will be allowed to make cross-domain calls. Consider network topology and any authentication mechanisms that will be affected by the configuration or implementation of the cross-domain policy.