Error message when you visit a Web site that is hosted on IIS 7.0: "HTTP Error 401.4 – Authorization failed by filter"

You have a Web site that is hosted on Internet Information Services (IIS) 7.0. When you visit this Web site, you may receive an error message that resembles the following:

This problem may occur if an ISAPI filter that is running on the Web server causes the authentication process to fail. This problem may also occur if the ISAPI filter denies the IIS request.

To resolve this problem, follow these steps:

1. Find out which ISAPI filter causes the problem. To do this, follow these steps to access the Failed Request Event Buffer (FREB) XML file of the Web site that generates the 401.4 error message:
   1. Locate the FREB XML file for the request that you made to the server. The FREB XML file is typically located in the following folder:
      drive:\inetpub\logs\FailedReqLogFiles
   2. Use Windows Internet Explorer to open the FREB XML file. Typically, the name of the FREB XML file resembles the following:
      frXXXXXX.xml
      Note The XXXXXX placeholder represents the FREB ID.
   3. In the FREB XML file, click the Request Details tab, and then expand the FILTER_START node. You can now see the physical path of the ISAPI filter that causes the problem.
2. Contact the developer of this ISAPI filter to obtain the resolution.