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System.Web.HttpException in Internet Information Services

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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
Original KB number:   2605401

Symptoms

Consider the following scenario:

  • You run a web application configured to use the Integrated Pipeline on a Microsoft Internet Information Services (IIS) 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 might occur:

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

Cause

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 isn't expected to change, and therefore ASP.NET throws the exception.

This exception isn't thrown if the application pool is running in Classic mode instead of Integrated Pipeline mode.

Resolution

To prevent this exception, use one of the following methods:

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

More information

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