HKEY_USERS is a Windows registry area that exposes the loaded user hives for the current session. Each subkey typically maps to a security identifier (SID) and represents the registry-backed settings for a user who is signed in or whose hive has been loaded by the system or an administrator.
This key matters because it shows which user profiles are active and how their settings are mounted into the registry namespace. In attacks, abuse of profile loading or hive-mounting can make another account’s registry state available under HKEY_USERS, which may help an attacker read, modify, or impersonate per-user configuration. In defense, security teams watch for unexpected hive loads, unusual SID entries, and registry activity that crosses account boundaries, since those patterns can indicate misuse of legitimate Windows profile mechanics rather than obvious malware.