Why do I need this model?
Because Active Directory is exposed, and don’t misunderstand this. It is exposed to persons, applications, services and networks, so there is a real risk to get it compromised. There are hundreds of details to take into account to objectively check the risk, but the risk is there, and the best thing to do is to manage the risk.
In the old times, just by having an antivirus was enough (Huuh!) to be protected. No internet available. Networks where pretty small, or even inexistent. As the devices where expensive, granting access to anyone was not so easy, so physical security was kind of present.
Today, we have an extreme connectivity, with at least 1 antivirus, anti-Trojans, worms, rootkits, spam… we have access control list for disks, groups, shares, mailboxes… we have many web applications and services, which use any kind of authentication and authorization… databases… social networks… BYOD… AND we have to manage it, assuring the integrity and security of all these. We cannot afford the risk of exposing all this information, or even worst compromising it.
We care about firewalls… networks… IDS… personal FW… antivirus… Authentication… Authorization… so: Why we are not protecting one of the most valuable and critical assets?
This model will not be the “ultimate” security for AD, but will help mitigate credential theft techniques.