System.Web.HttpException in Internet Information Services

This article helps you resolve the System.Web.HttpException that occurs when an application runs in Integrated Pipeline mode, and provides a resolution.

Original product version:   Internet Information Services 7.0 and later versions
Original KB number:   2605401


Consider the following scenario:

  • You run a web application configured to use the Integrated Pipeline on a Microsoft Internet Information Services (IIS) 7.0 or later versions web server.
  • The application uses an ISAPI filter registered for the SF_NOTIFY_AUTHENTICATION event.
  • The application's global.asax implements the Application_BeginRequest event.

Under these conditions, this exception may occur:

System.Web.HttpException: This server variable cannot be modified during request execution.


This behavior is by design.

When IIS calls the ISAPI filter for the SF_NOTIFY_AUTHENTICATION event, it updates the AUTH_USER server variable. Since the application is configured to use the Integrated Pipeline, ASP.NET looks for changes in server variables. AUTH_USER is one of the variables that is not expected to change, and therefore ASP.NET throws the above exception.

This exception will not get thrown if the application pool is running in Classic mode instead of Integrated Pipeline mode.


Either of the methods below prevents this exception:

  1. Configure the application pool to run in Classic mode instead of Integrated Pipeline mode.
  2. Use native or managed Http Modules to handle authentication notifications.

More information

For more information, see Develop a Native C\C++ Module for IIS 7.0.