Rich Internet Applications (RIA) have adopted Adobe's crossdomain.xml policy files to allow for controlled cross domain access to data and service consumption using technologies such as Oracle Java, Silverlight, and Adobe Flash. Therefore, a domain can grant remote access to its services from a different domain. However, often the policy files that describe the access restrictions are poorly configured. Poor configuration of the policy files enables Cross-site Request Forgery attacks, and may allow third parties to access sensitive data meant for the user.
A cross-domain policy file specifies the permissions that a web client such as Java, Adobe Flash, Adobe Reader, etc. use to access data across different domains. For Silverlight, Microsoft adopted a subset of the Adobe's crossdomain.xml, and additionally created it's own cross-domain policy file: clientaccesspolicy.xml.
Whenever a web client detects that a resource has to be requested from other domain, it will first look for a policy file in the target domain to determine if performing cross-domain requests, including headers, and socket-based connections are allowed.
Master policy files are located at the domain's root. A client may be instructed to load a different policy file but it will always check the master policy file first to ensure that the master policy file permits the requested policy file.
Most RIA applications support crossdomain.xml. However in the case of Silverlight, it will only work if the crossdomain.xml specifies that access is allowed from any domain. For more granular control with Silverlight, clientaccesspolicy.xml must be used.
Policy files grant several types of permissions:
An example of an overly permissive policy file:
<?xml version="1.0"?> <!DOCTYPE cross-domain-policy SYSTEM "http://www.adobe.com/xml/dtds/cross-domain-policy.dtd"> <cross-domain-policy> <site-control permitted-cross-domain-policies="all"/> <allow-access-from domain="*" secure="false"/> <allow-http-request-headers-from domain="*" headers="*" secure="false"/> </cross-domain-policy>
Testing for RIA policy files weakness:
To test for RIA policy file weakness the tester should try to retrieve the policy files crossdomain.xml and clientaccesspolicy.xml from the application's root, and from every folder found.
For example, if the application's URL is http://www.owasp.org, the tester should try to download the files http://www.owasp.org/crossdomain.xml and http://www.owasp.org/clientaccesspolicy.xml.
After retrieving all the policy files, the permissions allowed should be be checked under the least privilege principle. Requests should only come from the domains, ports, or protocols that are necessary. Overly permissive policies should be avoided. Policies with "*" in them should be closely examined.
<cross-domain-policy> <allow-access-from domain="*" /> </cross-domain-policy>