How to fix error "400 Bad Request - Request Header Or Cookie Too Large"?

If you encounter the error “400 Bad Request - Request Header Or Cookie Too Large” when trying to log in to Windows or via RDP, it may be due to a large number of cookies. Here’s how you can resolve this issue:

1. Go to the following Windows registry path: Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Rublon\WindowsLogon and set DebugRublonRequests to 1.

2. Perform a Windows/RDP login. A developer window will appear next to the Rublon Prompt.

3. In the developer window, navigate to the Application → Cookies tab and click the Clear all button.

4. After clearing the cookies, your next MFA login should proceed without any issues.

5. After fixing the 400 Bad Request error, disable the developer window by setting the DebugRublonRequests option back to 0. This will prevent the developer window from appearing for every user.

By following these steps, you should be able to resolve the “400 Bad Request - Request Header Or Cookie Too Large” error. If you continue to experience issues, contact Rublon Support.

Helpful Links

Rublon MFA for Windows Logon and RDP - Documentation