[asp.net] IIS AppPoolIdentity and file system write access permissions

Here's an issue with IIS 7.5 and ASP.NET that I've been researching and getting nowhere with. Any help would be greatly appreciated.

My question is: using ASP.NET in IIS 7.5, how does IIS and/or the operating system allow the web application to write to a folder like C:\dump when running under full trust? How is it that I don't have to explicitly add write access for the application pool user (in this case ApplicationPoolIdentity)?

This much I know:

• In IIS 7.5, the default Identity for an Application Pool is ApplicationPoolIdentity.
• ApplicationPoolIdentity represents a Windows user account called "IIS APPPOOL\AppPoolName", which is created when the Application Pool is created, where AppPoolName is the name of the Application Pool.
• The "IIS APPPOOL\AppPoolName" user is by default a member of the IIS_IUSRS group.
• If you are running under Full Trust, your web application can write to many areas of the file system (excluding folders like C:\Users, C:\Windows, etc). For example, your application will have access to write to some folders, like, C:\dump.
• By default, the IIS_IUSRS group is not given read or write access to C:\dump (at least not access that is visible through the "Security" tab in Windows Explorer).
• If you deny write access to IIS_IUSRS, you will get a SecurityException when trying to write to the folder (as expected).

So, taking all of that into account, how is write access granted to the "IIS APPPOOL\AppPoolName" user? The w3wp.exe process runs as this user, so what allows this user to write to a folder it doesn't seem to have explicit access to?

Please note that I understand this was probably done for the sake of convenience, since it would be a pain to grant a user access to every folder it needs to write to if you are running under Full Trust. If you want to limit this access, you can always run the application under Medium Trust. I am interested in finding out about the way the operating system and/or IIS allows these writes to take place, even though there appears to be no explicit file system access granted.

Each application pool in IIs creates its own secure user folder with FULL read/write permission by default under c:\users. Open up your Users folder and see what application pool folders are there, right click, and check their rights for the application pool virtual account assigned. You should see your application pool account added already with read/write access assigned to its root and subfolders.

So that type of file storage access is automatically done and you should be able to write whatever you like there in the app pools user account folders without changing anything. That's why virtual user accounts for each application pool were created.

I tried this to fix access issues to an IIS website, which manifested as something like the following in the Event Logs ? Windows ? Application:

Log Name:      Application
Source:        ASP.NET 4.0.30319.0
Date:          1/5/2012 4:12:33 PM
Event ID:      1314
Task Category: Web Event
Level:         Information
Keywords:      Classic
User:          N/A
Computer:      SALTIIS01

Description:
Event code: 4008 
Event message: File authorization failed for the request. 
Event time: 1/5/2012 4:12:33 PM 
Event time (UTC): 1/6/2012 12:12:33 AM 
Event ID: 349fcb2ec3c24b16a862f6eb9b23dd6c 
Event sequence: 7 
Event occurrence: 3 
Event detail code: 0 

Application information: 
    Application domain: /LM/W3SVC/2/ROOT/Application/SNCDW-19-129702818025409890 
    Trust level: Full 
    Application Virtual Path: /Application/SNCDW 
    Application Path: D:\Sites\WCF\Application\SNCDW\ 
    Machine name: SALTIIS01 

Process information: 
    Process ID: 1896 
    Process name: w3wp.exe 
    Account name: iisservice 

Request information: 
    Request URL: http://webservicestest/Application/SNCDW/PC.svc 
    Request path: /Application/SNCDW/PC.svc 
    User host address: 10.60.16.79 
    User: js3228 
    Is authenticated: True 
    Authentication Type: Negotiate 
    Thread account name: iisservice 

In the end I had to give the Windows Everyone group read access to that folder to get it to work properly.

1. Right click on folder.

2. Click Properties

3. Click Security Tab. You will see something like this:

4. Click "Edit..." button in above screen. You will see something like this:

5. Click "Add..." button in above screen. You will see something like this:

6. Click "Locations..." button in above screen. You will see something like this. Now, go to the very of top of this tree structure and select your computer name, then click OK.

7. Now type "iis apppool\your_apppool_name" and click "Check Names" button. If the apppool exists, you will see your apppool name in the textbox with underline in it. Click OK button.

8. Check/uncheck whatever access you need to grant to the account

9. Click Apply button and then OK.