Insecure crossdomain.xml policy
Description
The crossdomain.xml policy file controls whether Flash and other Adobe clients can access resources across different domains, bypassing the browser's same-origin policy. This server's crossdomain.xml file uses wildcard permissions (domain="*"), allowing any external domain to read data from this server. While this configuration may be acceptable for purely public content servers, it creates security risks for applications that handle sensitive data, use authentication mechanisms, or reside behind corporate firewalls. The overly permissive policy effectively disables cross-domain access controls that are designed to protect user data and prevent unauthorized cross-site interactions.
Remediation
Replace the wildcard (*) domain specification with an explicit allowlist of trusted domains that have a legitimate business need for cross-domain access. If cross-domain access is not required, remove the crossdomain.xml file entirely or deny all access. For applications requiring cross-domain functionality, implement a restrictive policy that specifies exact domains:
<cross-domain-policy> <site-control permitted-cross-domain-policies="master-only"/> <allow-access-from domain="trusted-domain.example.com" secure="true"/> <allow-access-from domain="another-trusted.example.com" secure="true"/> </cross-domain-policy>
Set the
secure="true" attribute to restrict access to HTTPS connections only. Use the site-control element to prevent policy files in subdirectories from overriding the master policy. Regularly review and audit the list of permitted domains. For modern applications, consider migrating away from Flash-based cross-domain requests to CORS (Cross-Origin Resource Sharing), which provides more granular and secure control over cross-origin access.