File.Exists with File/Dir Permissions

Posted on: February 22, 2016 1:48:16 AM
While attempting to troubleshoot a failing unit test on my work machine that didn't fail on my peer's machine or on the build machine I came across an interesting situation. Basically, I found the only difference between my code base and everyone else's was it's location on my computer. Mine was in my Windows user directory whereas everyone else's existed in custom folders directly off the root directory. I began to investigate permissions because of this scenario and what I found was very interesting and a bit misleading around System.IO.File.Exists. The MSDN Documenation for File.Exists says, "If the caller does not have sufficient permissions to read the specified file, no exception is thrown and the method returns false regardless of the existence of path." Based on this line, I wrote out a simple test program:
using System;
using System.DirectoryServices;
using System.IO;
using System.Runtime.InteropServices;
using System.Security.AccessControl;
using System.Security.Principal;

namespace ConsoleApplication1
{
    internal class Program
    {
        private const string DirName = "TestDir";
        private const string FileName = "File.txt";
        private const string Password = "Password1";
        private const string UserName = "PermissionTestUser";
        private static WindowsImpersonationContext Identity = null;
        private static IntPtr LogonToken = IntPtr.Zero;

        public enum LogonProvider
        {
            LOGON32_PROVIDER_DEFAULT = 0,
            LOGON32_PROVIDER_WINNT35 = 1,
            LOGON32_PROVIDER_WINNT40 = 2,
            LOGON32_PROVIDER_WINNT50 = 3
        };

        public enum LogonType
        {
            LOGON32_LOGON_INTERACTIVE = 2,
            LOGON32_LOGON_NETWORK = 3,
            LOGON32_LOGON_BATCH = 4,
            LOGON32_LOGON_SERVICE = 5,
            LOGON32_LOGON_UNLOCK = 7,
            LOGON32_LOGON_NETWORK_CLEARTEXT = 8, // Win2K or higher
            LOGON32_LOGON_NEW_CREDENTIALS = 9 // Win2K or higher
        };

        public static void Main(string[] args)
        {
            string filePath = Path.Combine(DirName, FileName);
            try
            {
                CreateUser();
                CreateDir();
                CreateFile(filePath);

                // grant user full control to the dir
                SetAccess(DirName, AccessControlType.Allow);
                // deny user full control to the file
                SetAccess(filePath, AccessControlType.Deny);

                // impersonate user
                Impersonate();
                Console.WriteLine("File.Exists (with dir permissions): {0}", File.Exists(filePath));
                UndoImpersonate();

                // deny access to dir
                SetAccess(DirName, AccessControlType.Deny);

                // impersonate user
                Impersonate();
                Console.WriteLine("File.Exists (without dir permissions): {0}", File.Exists(filePath));
                UndoImpersonate();
            }
            finally
            {
                UndoImpersonate();
                DeleteDir();
                DeleteUser();
            }
        }

        [DllImport("kernel32.dll", CharSet = CharSet.Auto)]
        private static extern bool CloseHandle(IntPtr handle);

        private static void CreateDir()
        {
            Directory.CreateDirectory(DirName);
        }

        private static void CreateFile(string path)
        {
            File.Create(path).Dispose();
        }

        private static void CreateUser()
        {
            DirectoryEntry ad = new DirectoryEntry("WinNT://" + Environment.MachineName + ",computer");
            DirectoryEntry newUser = ad.Children.Add(UserName, "user");
            newUser.Invoke("SetPassword", new object[] { Password });
            newUser.Invoke("Put", new object[] { "Description", "Test user" });
            newUser.CommitChanges();
        }

        private static void DeleteDir()
        {
            Directory.Delete(DirName, true);
        }

        private static void DeleteUser()
        {
            DirectoryEntry ad = new DirectoryEntry("WinNT://" + Environment.MachineName + ",computer");
            DirectoryEntries users = ad.Children;
            DirectoryEntry user = users.Find(UserName, "user");

            if (user != null)
            {
                users.Remove(user);
            }
        }

        private static void Impersonate()
        {
            if (LogonUser(UserName, ".", Password, (int)LogonType.LOGON32_LOGON_INTERACTIVE, (int)LogonProvider.LOGON32_PROVIDER_DEFAULT, ref LogonToken))
            {
                Identity = WindowsIdentity.Impersonate(LogonToken);
                return;
            }
        }

        [DllImport("advapi32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
        private static extern bool LogonUser(string lpszUserName,
            string lpszDomain,
            string lpszPassword,
            int dwLogonType,
            int dwLogonProvider,
            ref IntPtr phToken);

        private static void SetAccess(string path, AccessControlType type)
        {
            FileSecurity fs = File.GetAccessControl(path);
            FileSystemAccessRule far = new FileSystemAccessRule(UserName, FileSystemRights.FullControl, type);
            fs.AddAccessRule(far);
            File.SetAccessControl(path, fs);
        }

        private static void UndoImpersonate()
        {
            if (Identity != null)
            {
                Identity.Undo();
                Identity = null;
            }

            if (LogonToken != IntPtr.Zero)
            {
                CloseHandle(LogonToken);
                LogonToken = IntPtr.Zero;
            }
        }
    }
}
This program basically tests file permissions and folder permissions with File.Exists. What I expected to see based on the documentation was both instances should return false; however, I found that when the user has permissions on the directory but none on the file, it still returns true. This ends up being due to a very subtle keyword in the documentation, "sufficient". If the user has the LIST permission in the directory the file lives in, File.Exists has sufficient permissions to return whether or not the file exists.

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