Many IIS FTP developers may not have noticed, but all custom FTP extensibility providers execute through COM+ in a DLLHOST.exe process, which runs as NETWORK SERVICE by default. That being said, NETWORK SERVICE does not always have the right permissions
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Read the complete post at http://blogs.iis.net/robert_mcmurray/archive/2011/12/01/changing-the-identity-of-the-ftp-extensibility-process.aspx