For years Microsoft has stated that the forest was the security boundary in Active Directory. For example, Microsoft’s “What Are Domains and Forests?” document (last updated in 2014) has a “Forests as Security Boundaries” section which states (emphasis added): Each forest is a single instance of the directory, the top-level Active Directory container, and a security boundary for all objects that are located in the forest. This security boundary defines the scope of authority of the administrators. In general, a security boundary is defined by the top-level container for which no administrator external to the container can take control away…