In previous versions of IIS, the IUSR_MachineName account is created for anonymous authentication. This is an actual user account created on the server (a domain account can be used in domain environments), and like all user accounts— it has a SID, and an account password with the accompanying management costs and risks.
One of the resulting annoyances (for me): when you install IIS first and then change the computer name, the computer name and the MachineName in IUSR_MachineName account don’t match.
IIS 7 gets rid of the IUSR_MachineName account in favor of a built-in IUSR account that’s guaranteed to have the same SID on all computers. This ensures ACLs copied from one web server to another work, domain accounts are no longer required, and applications can be easily deployed across multiple web servers. The IIS_WPG group (for IIS Application Pool identities) is replaced by the built-in group IIS_IUSRS.
Note: The IUSR_MACHINENAME account isn’t completely gone— it is used for anonymous authentication to FTP, and gets created if/when you install FTP.
More on the IIS team blog in ‘Understanding the Built-In User and Group Accounts in IIS 7.0‘
– Security identifiers
– Well-known security identifiers in Windows operating systems
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