The X-Permitted-Cross-Domain-Policies header plays an important role in managing how a website shares its resources with other domains. This header is particularly relevant for sites that host Flash or PDF files, which can load data from various sources. It controls if and how files on a domain can be used by other domains, thereby providing an additional layer of security against abuse of resource-sharing policies.
The X-Permitted-Cross-Domain-Policies header can take several values, each specifying a different level of permission for cross-domain resource sharing:
1. **None**: No external domain can use the site’s resources. This is the most restrictive setting and is used to completely block cross-domain sharing.
2. **Master-only**: Only the main crossdomain.xml file (located at the root of the site) is considered for cross-domain sharing policies.
3. **By-content-type**: Allows cross-domain sharing only for files served with an explicit MIME type for Flash or PDF files.
4. **All**: Allows all crossdomain.xml files on the site to specify cross-domain sharing policies.
Using this header is essential to prevent attacks where a site’s resources can be abusively embedded or used by third-party sites without authorization. For example, without appropriate restrictions, a malicious site could embed a Flash file from another site and interact with it in a way that compromises security or confidentiality.
By carefully configuring the X-Permitted-Cross-Domain-Policies header, website administrators can ensure that their resources are not inappropriately used by other sites, thus helping to maintain the integrity and security of their content and that of their users.