What Is Hku S-1-5-21?

HKU S-1-5-21 is a unique identifier that is used to represent a user account in Microsoft Windows operating systems. The abbreviation HKU stands for “HKEY_USERS,” which is a registry key that contains information about every user account created on a Windows computer. The S-1-5-21 part of the identifier is a security identifier (SID) that Windows uses to uniquely identify each user account.

When a user logs into a Windows computer, Windows assigns them a unique SID that is used to determine their access rights to various system resources such as files, folders, and registry keys. The HKU S-1-5-21 identifier is used by Windows to identify the user profile associated with a particular user account. This user profile contains the user’s personal settings, preferences, and other user-specific data that is stored on the computer’s hard drive. Understanding how HKU S-1-5-21 works is important for system administrators and users who need to manage user accounts and configure user profiles on a Windows computer.

What is HKU S-1-5-21?

– HKU S-1-5-21 is a registry key that is used on Windows operating systems.
– It is a unique identifier that refers to a user profile on the system, and is created when a user logs onto the system for the first time.
– The numerical portion (S-1-5-21) represents the security identifier (SID) that is assigned to all user profiles on a Windows system.
– The HKU portion refers to the HKEY_USERS registry hive, which contains the user profiles on the system.
– In other words, HKU S-1-5-21 is a way to reference a specific user profile in the Windows Registry.


1. What is HKU S-1-5-21?
HKU S-1-5-21 is a unique identifier for a user account on a Windows operating system. It is also known as the Security Identifier (SID) for the user’s account.

2. How is HKU S-1-5-21 generated?
HKU S-1-5-21 is generated by Windows during the creation of a user account. It is based on the System Identifier (SID) of the computer and a unique identifier for the user account.

3. What is the purpose of HKU S-1-5-21?
HKU S-1-5-21 is used by Windows to manage security and access to resources on the computer. It is a part of the security mechanism in place to ensure that only authorized users can access files, folders, and other resources.

4. Can HKU S-1-5-21 be changed?
No, HKU S-1-5-21 cannot be changed once it has been generated. It is a system-generated identifier that cannot be modified by users.

5. Is HKU S-1-5-21 unique to each user account on a computer?
Yes, HKU S-1-5-21 is unique to each user account on a computer. It is used to differentiate between different user accounts and to ensure that each account is granted the appropriate level of access and security permissions.


In conclusion, understanding the intricate details of HKU S-1-5-21 and its significance is an essential aspect of managing and securing your Windows operating system. From managing user profiles, permissions, and registry keys, everything in the system revolves around this unique identifier. By learning more about HKU S-1-5-21, you can effectively troubleshoot Windows issues, improve system performance, and fortify your system against harmful cyber threats. So, take the time to gain a better understanding of this critical component and ensure that your system remains safe and secure.

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