Applicationhost.config – cannot write configuration due to insufficient permission

I encountered this problem too this afternoon.

I did a quick google and found others with the same issue on the asp.net forum. The issue seemed to be that

Here is what got me past this error, hope it works for you.

the runtime tries to load the corresponding configuration, it doesn't take the bitness of the application pool into the consideration. As you know, ASP.NET 1.1 is only supported on 32-bit. So if you have a 64-bit OS, in order to run ASP.NET 1.1, you have to enable 32-bit applications in the application pool. So, with the QFE/SP2 on 64-bit OS (and only on 64-bit OS), the runtime is incorrectly looking for the 1.1 version of the configuration under Framework64, which does not exist.

Workaround:

1. Create the Framework64 directory for 1.1 md \windows\microsoft.net\framework64\v1.1.4322\config\
2. Copy the 32bit config to 64bit config location created in step 1. copy \windows\microsoft.net\framework\v1.1.4322\config\machine.config \windows\microsoft.net\framework64\v1.1.4322\config\

http://blogs.iis.net/wonyoo/archive/2009/06/18/workaround-running-asp-net-1-1-on-vista-sp2-ws08-sp2.aspx

There is no problem with your web.config. Your web site runs under a process. In iis you can define the identity of that process. The identity that your web site's application pool runs as (Network Services, Local System, etc.), should have permission to access and read web.config file.

Update:

This updated answer is same as above, but a little longer and simpler and improved.

First of all: you don't have to change anything in your config file. It's OK. The problem is with windows file permissions.

This problems occurs because your application can not access and read web.config file.

Make the file accessible to IIS_IUSRS group. Just right click web.config and click properties, under security tab, add IIS_IUSRS.

So what is this IIS_IUSRS thing?

Your web site is like an exe file. Just like any exe file, it should be started by a user and it runs according to permissions assigned to that user.

When your site is started in IIS, Application Pool of your web site is associated with a user (Network Services, Local System, Etc. ...) (and can be changed in IIS)

So when you say IIS_IUSRS, it means any user (Network Services, Local System, Etc. ...) that your site is running as.

And as @Seph mentioned in comment below: If your computer is on a domain, remember that IIS_IUSRS group is a local group. Also make sure that when you're trying to find this user check the location it should be set to local computer and not a corporate domain.