UsrClass.dat is the Windows file that stores a user’s per-account Classes registry data. It helps define settings such as file associations, COM registrations, and other entries that contribute to the user-specific view of HKEY_CLASSES_ROOT. Windows loads this file as part of the user profile, so it is separate from machine-wide registry data and is meant to preserve account boundaries.
This matters in security because attackers may target profile loading rather than direct registry editing. If a low-privilege process can influence how UsrClass.dat is mounted, it may be able to alter another user’s settings or cause changes that take effect later when that user signs in. Defenders watch for unexpected hive load failures, profile-service errors, and suspicious activity under per-user Classes paths, since abuse of this file can indicate local privilege escalation or delayed configuration tampering.