We really want to make sure that code running in a DNN website cannot access anything in C:\ no matter what. By default all Application Pool Accounts have some limited permissions which are not needed, so we want to ensure that these cannot be used either. 

1. On the web server, open the permissions for the drive C:\
2. Add Deny permissions for the IIS_Users group (which contains all automatically generated Application Pool Identities) like this:
3. You'll receive warnings about the following objects, where the Deny cannot be applied. This is ok.
   1. c:\pagefile.sys
   2. c:\Program Files
   3. c:\Program Files (x86)
   4. c:\Windows

Note that Windows will automatically manage the permissions in C:\Windows etc. as needed, so you don't need to make changes there, unless you are using a setup where the web site files are stored on a remote file store (NAS). 

Special Cases

In case you're also hosting your website files on drive C (which you really shouldn't, they should be on another drive), remember to remove this newly added Deny-permission on those folders and harden that folder according to best practices.