The Flash cross-domain policy controls whether Flash client components running on other domains can perform two-way interaction with the domain that publishes the policy. If another domain is allowed by the policy, then that domain can potentially attack users of the application. If a user is logged in to the application, and visits a domain allowed by the policy, then any malicious content running on that domain can potentially gain full access to the application within the security context of the logged in user.
Even if an allowed domain is not overtly malicious in itself, security vulnerabilities within that domain could potentially be leveraged by a third-party attacker to exploit the trust relationship and attack the application that allows access. Any domains that are allowed by the Flash cross-domain policy should be reviewed to determine whether it is appropriate for the application to fully trust both their intentions and security posture.
Any inappropriate entries in the Flash cross-domain policy file should be removed.