Fix 0xC0000084: Invalid ID Authority Error

This error shows up when Windows can't validate an account SID. Usually from a corrupted registry or broken security identifier. Here's how to kill it.

What Causes 0xC0000084?

You'll see this error right after Windows tries to load your desktop. It hangs, then pops up “STATUS_INVALID_ID_AUTHORITY”. The culprit here is almost always a corrupted security identifier (SID) in your user profile. This can happen after a failed Windows update, a disk error, or if you messed with the registry manually.

Fix 1: Quick 30-Second Check

Before you dive into anything deep, try this. It won't fix the root cause, but it gets you back in fast.

Restart your PC. Hold Shift while clicking Restart. This gets you into the recovery environment.
Choose Troubleshoot > Advanced Options > Startup Settings > Restart.
Press 4 to boot into Safe Mode.

If you can log in safely, your main profile is trashed. Skip to Fix 2 below. If Safe Mode shows the same error, you've got a system-level problem. Go straight to Fix 3.

Fix 2: The 5-Minute Registry Fix

This works 80% of the time. We're going to delete the broken SID cache for your user account.

Boot into Safe Mode (as above). Log in with a built-in admin account (like Administrator).
Press Win + R, type regedit, hit Enter.

Go to:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList

Look for a key that ends with “.bak”. Also look for keys with long SID strings (like S-1-5-21-...). Right-click each key, choose Export to back it up first.
Delete the key that has a “.bak” extension AND the key without .bak that matches the same SID. Usually there's a pair: one with .bak, one without. Delete both.
Close regedit, restart normally.

This forces Windows to rebuild your profile on next login. Your desktop and files stay intact in the old profile folder under C:\Users\[oldname]. You might need to copy them over.

Fix 3: The 15+ Minute Deep Clean

If Fix 2 didn't work, your NTUSER.DAT file is probably corrupt. This is the registry hive for your user account.

Boot from a Windows installation USB (or recovery drive). Choose Repair your computer > Troubleshoot > Command Prompt.

Type:

ren C:\Users\[YOURUSERNAME]\NTUSER.DAT NTUSER.old

Then:

copy C:\Users\Default\NTUSER.DAT C:\Users\[YOURUSERNAME]\NTUSER.DAT

Reboot.

This replaces the bad hive with a fresh default one. Your old settings are gone, but you won't lose files.

Still Broken? Try This

If none of that works, create a new local admin account from Safe Mode:

In Safe Mode, right-click Start > Computer Management > Local Users and Groups > Users.
Right-click blank area > New User. Give it a name like “TempAdmin”, set a password.
Right-click the new user > Properties > Member Of > Add > type “Administrators” > OK.
Restart, log into TempAdmin. Copy your files from C:\Users\[oldname] to the new profile.

Do not bother with SFC or DISM scans here — they rarely fix this specific error. The problem is profile-specific, not system file corruption.

Final tip: If you see 0xC0000084 on a domain-joined machine, check the domain controller's SID authority. But for home users, it's almost always a local profile nuke. You're welcome.