That is by what I know correct. If you do not impersonate as the calling user, by default it should run under the default identity from the application pool.
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Below are some good links you can go through these to understand the iis user
In summary:
- IIS 5.1 (Windows XP) - it's a local ASPNET account
- IIS 6 - by default, application pools run under the Network Service account which is a member of IIS_WPG group
- IIS 7.0 - still Network Service
- IIS 7 SP2 / 7.5 - runs under an ephemeral thing called ApplicationPoolIdentity which maps to an account IIS APPPOOL\ApplicationPool (e.g. IIS APPPOOL\DefaultAppPool). This account is a member of IIS_IUSRS user group
Infomation source:-
User ASP.NET runs under What are all the user accounts for IIS/ASP.NET and how do they differ?
http://www.bluevisionsoftware.com/WebSite/TipsAndTricksDetails.aspx?Name=AspNetAccount
Source: Shekhar - In which Account By Default ASP.NET 4.0 Code Execute?