This article explains how to perform debugging of an ASP.NET application that runs in the same site as Microsoft® SharePoint.

You may receive the following error message when trying to debug an ASP.NET application on the same machine where SharePoint is installed:
 
Error HTTPException (0x80004005) Debugging is not supported under current trust level settings
 

This error occurs when running ASP.NET web applications when “Debug” is set to "true".

To repair the error, add one of the following lines to the “Web.config” file, which is located in the ASP.NET application’s directory:

<trust level="WSS_Medium" originUrl="" />

or

<trust level="Full" originUrl="" />