HKEY_CLASSES_ROOT, often shortened to HKCR, is a merged Windows registry view that combines machine-wide class settings with the current user’s per-user Classes data. It is not a separate file-backed hive by itself; instead, Windows presents it as a unified view over the system Classes information and the user’s own Classes registrations.
This matters in security because HKCR influences file associations, COM object registration, and other application-handling behavior. Attackers may abuse these settings to hijack how Windows opens files or launches components, while defenders watch it for tampering that can persist across logons. In privilege-escalation cases, a flaw in profile loading or hive mounting can let a low-privilege user affect another account’s Classes data, which then appears through HKCR when that user logs in. That makes HKCR a boundary worth protecting: changes may look like ordinary configuration, but they can redirect execution or alter trusted behavior later.