The Windows 2012 DNS Servers zone files must not include CNAME records pointing to a zone with lesser security for more than six months.

The use of CNAME records for exercises, tests, or zone-spanning (pointing to zones with lesser security) aliases should be temporary (e.g., to facilitate a migration) and not be in place for more than six months. When a host name is an alias for a record in another zone, an adversary has two points of attack: the zone in which the alias is defined and the zone authoritative for the alias's canonical name. This configuration also reduces the speed of client resolution because it requires a second lookup after obtaining the canonical name. Furthermore, in the case of an authoritative name server, this information is promulgated throughout the enterprise to caching servers and thus compounds the vulnerability.