FIX: Error message when you configure Internet Explorer to use a .pac file and you try to access a Web site that requires Kerberos authentication: "HTTP Error 401"

Support for Windows XP has ended

Microsoft ended support for Windows XP on April 8, 2014. This change has affected your software updates and security options. Learn what this means for you and how to stay protected.

This article has been archived. It is offered "as is" and will no longer be updated.
Consider the following scenario:
  • You are running Microsoft Internet Explorer 6 on a computer that is running Microsoft Windows XP.
  • You configure Internet Explorer to use a proxy auto configuration (.pac) file.
  • You try to access a Web site that requires Kerberos authentication.
In this scenario, you may receive an error message that resembles the following:
HTTP Error 401 - Unauthorized: Access is denied due to invalid credentials.

Hotfix information

A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing this specific problem. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.

If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, contact Microsoft Customer Service and Support to obtain the hotfix.

Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site: Note The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.


The computer must be running Windows XP Service Pack 2 (SP2) to install this fix.

Restart requirement

You must restart the computer after you apply this hotfix.

Hotfix replacement information

This hotfix does not replace any other hotfixes.

Registry information

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756 How to back up and restore the registry in Windows
After you apply software update 921400, follow these steps to enable the update:
  1. Exit Internet Explorer.
  2. Click Start, click Run, type regedit, and then click OK.
  3. Locate and then click the following registry subkey:
    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\Main
  4. On the Edit menu, point to New, and then click Key.
  5. Type FeatureControl, and then press ENTER.
  6. Select the FeatureControl registry subkey that you created in step 5, point to New on the Edit menu, and then click Key.
  7. Type FEATURE_AUTOPROXY_CACHE_ANAME_KB921400, and then press ENTER.
  8. Select the FEATURE_AUTOPROXY_CACHE_ANAME_KB921400 registry subkey that you created in step 7, point to New on the Edit menu, and then click DWORD Value.
  9. Type Iexplore.exe, and then press ENTER.
  10. Right-click Iexplore.exe, and then click Modify.
  11. In the Value data box, type 1, and then click OK.

    Note To revert to the previous behavior for backward compatibility, reset this value to 0.
  12. Exit Registry Editor.

    Note The feature FEATURE_AUTOPROXY_CACHE_ANAME_KB921400 control key can also contain other applications, for example:
    • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\Main\FEATURE_AUTOPROXY_CACHE_ANAME_KB921400\Excel.exe
    • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\Main\FEATURE_AUTOPROXY_CACHE_ANAME_KB921400\Powerpnt.exe
If there are multiple applications, you can add a DWORD entry with the name of "*" and a value of 1 to activate this feature for all the installed applications that use URLs.

Hotfix file information

The English version of this hotfix has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time item in Control Panel.
File nameFile versionFile sizeDateTimePlatform
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
The problem does not occur if you do not use a .pac file or if you do not call a scripting API that causes a DNS resolution of the host that you access by using the .pac file.

Article ID: 921400 - Last Review: 01/16/2015 09:14:39 - Revision: 3.0

Microsoft Internet Explorer 6.0, Microsoft Windows XP Home Edition, Microsoft Windows XP Professional

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