.NET Security in C#

Since my company Harrissoft.co.uk does a lot of .NET consultancy, one of our recent projects required that file I/O access be denied if the user running the application did not have administrator privileges. A lot has been written about the command line utility caspol.exe, however, this can seem a little over the top and quite complex when considering code groups, policy levels and zone management.

I basically wanted to programmatically check whether the user had the relevant permissions by accessing their windows account. Fortunately, .NET provides this through the System.Security.Principal namespace. I also wanted to deny access to particular drives - this is done through the namespace System.Security.Permissions.

Below is shown a skeleton example, where if the user is not an administrator the contents of a text file cannot be read and displayed in a list box:

try
{
//By default deny access to the C Drive.....
CodeAccessPermission UserPermission = new FileIOPermission(FileIOPermissionAccess.AllAccess,@"c:\");
//Check whether the user is part of the administrator group
AppDomain.CurrentDomain.SetPrincipalPolicy(PrincipalPolicy.WindowsPrincipal);
WindowsPrincipal principal = (WindowsPrincipal)Thread.CurrentPrincipal;
WindowsIdentity identity = (WindowsIdentity)principal.Identity;
bIsAdmin = principal.IsInRole(WindowsBuiltInRole.Administrator);
//Its not, so deny access to the file
if(!bIsAdmin)
{
UserPermission.Deny();
}
else
{
//Do the read
din = ReadTheFile.DoTheRead();
}
if(!bIsAdmin)
{
//Reset deny permissions in current stack frame
CodeAccessPermission.RevertDeny();
}
//If we got this far .... we read in the file
String str;
while ((str=din.ReadLine()) != null)
{
listBox1.Items.Add(str);
}
}
catch (SecurityException exception)
{
//Failed to pass the security checks - so flag up error to user
listBox1.Items.Add("Permission denied accessing file");
}

The zip download file contains the .NET project so you can build and run this example.