An unexpected 401.1 status is returned when you use Pre-Authentication headers with Internet Explorer and IIS

This article resolves the problem where an unexpected 401.1 status is returned with Pre-Authentication headers. It occurs when you use Internet Explorer to browser to a web application hosted on Internet Information Services (IIS).

Original product version: IE mode for Edge, Internet Information Services, Internet Explorer 11, 10, 9
Original KB number: 2749007


Consider the following scenario:

  • You use Windows Internet Explorer to browse to a web application hosted on IIS 7.0 or higher.
  • The Internet Explorer browser is configured to use Pre-Authentication, and Kernel Mode Authentication is enabled in IIS.
  • Additionally, this web request being sent by Internet Explorer is the first request to be sent to the IIS application.

In this scenario, IIS may return an HyperText Transfer Protocol (HTTP) 401.1 response to Internet Explorer in response to the browser's request. The web browser may prompt you to enter your username and password. Or, the HTTP 401.1 error message may be displayed in the browser window.


This behavior is by design. The 401.1 response will occur if the web browser's first request that's sent to the IIS application contains one of the following headers:

  • a Windows Challenge/Response (NTLM) header
  • a Negotiate WWW-Authorization header (known as Pre-Authentication)


There are many reasons a user may be prompted for credentials in Internet Explorer that are outside the scope of this article. See the More information section below to learn how to determine if the cause of the prompt is from the issue described here.


To work around this behavior, disable Pre-Authentication in Internet Explorer, or turn off Kernel Mode Authentication for the IIS Web application.


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To modify this behavior in Internet Explorer, use Registry Editor (Regedt32.exe) to add a value to the following registry key:
HKEY_CURRENT_USER/Software/Microsoft/Windows/CurrentVersion/Internet Settings/


The above registry key is one path; it has been wrapped for readability.

Add the following registry value:

  • Value Name: DisableNTLMPreAuth
  • Data Type: REG_DWORD
  • Value: 1

To modify this behavior in IIS, disable Kernel Mode Authentication for the IIS web application.

  1. Open Internet Information Services (IIS) Manager by running the following command from an administrative command prompt:

  2. In the Connections pane, expand the server name, expand Sites, and then the site, application, or Web service for which you want to disable Kernel Mode Authentication.

  3. Scroll to the Security section in the Home pane, and then double-click Authentication.

  4. In the Authentication pane, select Windows Authentication.

  5. Select Advanced Settings in the Actions pane.

  6. When the Advanced Settings dialog box appears, clear the Enable Kernel-mode authentication checkbox.

  7. Select OK to close the Advanced Settings dialog box.


Disabling Kernel Mode Authentication may cause web applications that require Kerberos authentication and delegation to fail.

More information

To determine if the prompt is caused by the issue described in this article, use the Fiddler tool. Use the tool to view the HTTP request/response traffic for the request resulting in the prompt in Internet Explorer. You'll also need the IIS logs from the IIS Server to confirm the HTTP status and sub status codes. The following example uses Internet Explorer 9 to illustrate this behavior:

  1. Start the Fiddler Tool and enable traffic capture.

  2. Browse to the IIS web application such that it will result in the prompt for credentials.

  3. In Fiddler, look for the request that resulted in the 401. Looking at the raw request and response views, you'll see entries similar to the following ones:

    Request Headers:

    GET /App1/default.aspx HTTP/1.1
    Accept-Language: en-US
    Accept-Encoding: gzip, deflate
    Connection: Keep-Alive
    Host: websitename
    Cookie: ASP.NET_SessionId=jdzbfpnmacq0jykhxnhqhe3j
    Authorization: Negotiate
    <header content omitted>

    Response Headers

    HTTP/1.1 401 Unauthorized
    Content-Type: text/html
    Server: Microsoft-IIS/7.5
    WWW-Authenticate: Negotiate
    WWW-Authenticate: NTLM
    X-Powered-By: ASP.NET
    Date: Wed, 22 Aug 2012 17:41:09 GMT
    Content-Length: 1293
    Proxy-Support: Session-Based-Authentication


The initial request to the web application already contains the Authorization header, which then results in the 401 response. The corresponding IIS log should show an entry similar to the following one:

2012-08-22 17:41:09 2001:4898:0:fff:200:5efe: GET /App1/default.aspx - 80 - 2001:4898:0:fff:0:
5efe: Mozilla/4.0+(compatible;+MSIE+7.0;+Windows+NT+6.1;+WOW64;+Trident/5.0;+SLCC2;+.NET+CLR+
RTC+EA+2;+BRI/1;+Zune+4.7;+MS-RTC+LM+8;+BRI/2;+Creative+AutoUpdate+v1.41.02) 401 1 2148074254 5005

The HTTP status and sub status are 401.1, which maps to Access Denied due to Invalid credentials.

For more information, see the following documentation: