What measures should be taken to prevent File System Object from accessing directories of other customers on an IIS shared web host?

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http://serverfault.com – We have our website on an IIS shared hosting server. While working on a file monitor for our site, we have found that the File System Object allows us to enumerate the directories and files of other customers that are on the same server as us. Attempting to enumerate files in some root customer directories results in access denied errors - but not all. Is this an IIS configuration error? A Windows configuration error? How can we direct our hosting company to fix the problem? (HowTos)