SPWeb.FirstUniqueAncestorSecurableObject property
SharePoint 2013
An ISecurableObject interface that represents the first unique ancestor website. This is the object where the ACL is defined. For a web with unique permission the First Unique Ancestor is itself. For a web that inherits permissions the First Unique Ancestor is one of the ancestral webs.
Assembly: Microsoft.SharePoint (in Microsoft.SharePoint.dll)