HKEY_CURRENT_USER is the Windows registry view for the account that is currently signed in. It is not a separate file-based hive itself; instead, Windows rebuilds this view at logon from the user’s profile hive and presents it as the active per-user configuration space. Applications use it for settings that should follow the logged-in user, such as preferences, shell behavior, and other session-specific data.
It matters in security because any change written here can affect what happens when that user starts a session. Attackers abuse this by planting malicious values, hijacking per-user startup behavior, or preparing registry state that is consumed later during sign-in. Defenders watch this area for unexpected writes, suspicious hive mounts, and profile-service activity, because tampering with HKEY_CURRENT_USER can become a delayed execution path when a privileged user logs on.