Figure 1

Article
07/03/2008

Figure 1 Security Check for MyFileAccessor's Constructor

  using System.Security.Permissions;
public class MyFileAccessor {
  public MyFileAccessor(String  path,
                        bool    readOnly) {
   path = MakeFullPath(path); // helper fcn
    FileIOPermissionAccess desiredAccess =
      readOnly ? FileIOPermissionAccess.Read :
                 FileIOPermissionAccess.AllAccess;
    FileIOPermission p =
      new FileIOPermission(desiredAccess, path);
    p.Demand();
    // 
      ••• 
      open the file
  }
  // •••
}

Figure 2 Code Access Permissions

Permission	Description
SecurityPermission	This is really a meta-permission, as it governs use of the security infrastructure itself. Several orthogonal permissions are grouped under this category: execution, assertion, unmanaged code, skip verification, serialization formatter, control domain policy, control evidence, control policy, control principal, and control thread.
FileIOPermission	Controls read, write, and append access to individual files and directory trees. Can also be used to restrict all access to the file system.
FileDialogPermission	Allows read-only access to files only if the file name is specified by the interactive user via a system-provided file dialog box. Typically used when FileIOPermission is not granted.
IsolatedStorage-FilePermission	Allows control over the usage of and lifetime configuration settings for isolated storage (as of this writing, the documentation for this permission is missing).
ReflectionPermission	Allows expansion of normal reflection privileges. With this permission, you can enumerate all types in an assembly and their contents, including private members. The details of this permission are still being ironed out.
RegistryPermission	Controls read, write, and create access to registry keys (including subkeys). Can also be used to restrict all access to the registry.
EnvironmentPermission	Controls read and write access to individual environment variables. Can also be used to restrict all access to an environment.
UIPermission	Can be used to restrict access to the clipboard (for instance, an application domain might only be able to paste data originating from the same application domain). Can also be used to restrict window usage to "safe" windows in an attempt to avoid attacks that mimic system dialog boxes that ask for sensitive information, such as passwords.

Figure 5 IsSubsetOf

  CodeAccessPermission p1 = new FileIOPermission(
  FileIOPermissionAccess.AllAccess,
  "c:\\temp");
CodeAccessPermission p2 = new FileIOPermission(
  FileIOPermissionAccess.Append,
  "c:\\temp\\foo\\bar.txt");

if (p1.IsSubsetOf(p2))
  System.Console.WriteLine("p1 is subset of p2");
else if (p2.IsSubsetOf(p1))
  System.Console.WriteLine("p2 is subset of p1");
else
  System.Console.WriteLine("no subset");

Figure 6 Setting Access Permissions

  using System.Security.Permissions;
   using System.Security;

public interface IUserPluggableAlgorithm {
  int Calculate(int a, int b);
}

// code snippet that uses the algorithm
PermissionSet ps =
  new PermissionSet(PermissionState.None);
ps.AddPermission(new FileIOPermission(
  FileIOPermissionAccess.AllAccess,
  "c:\\sensitiveStuff"));
ps.AddPermission(new FileIOPermission(
  FileIOPermissionAccess.AllAccess,
  "c:\\moreSensitiveStuff"));
ps.AddPermission(new EnvironmentPermission(
  PermissionState.Unrestricted));
ps.AddPermission(new UIPermission(
  PermissionState.Unrestricted));

ps.Deny();  // deny these permissions
int result = a.Calculate(42, 64);

CodeAccessPermission.RevertDeny();
// 
  ••• 
  continue doing work

Figure 7 Error Logging Service

  using System;
using System.IO;

public class ErrorLogger {
  public static void Log(String s) {
    const String fname = "c:\\temp\\errlog.txt";
    FileStream logStream = new FileStream(fname,
      FileMode.Append, FileAccess.Write);
    StreamWriter logWriter =
      new StreamWriter(logStream);
    logWriter.Write(s);
    logWriter.Close();
    logStream.Close();
  }
}

Figure 8 ErrorLogger2 Class

  public class ErrorLogger2 {
  public static void Log(String s) {
    const String fname = "c:\\temp\\errlog.txt";
    FileIOPermission p = new FileIOPermission(
      FileIOPermissionAccess.Append, fname);
    p.Assert();
    FileStream logStream = new FileStream(fname,
      FileMode.Append, FileAccess.Write);
    StreamWriter logWriter =
      new StreamWriter(logStream);
    logWriter.Write(s);
    logWriter.Close();
    logStream.Close();
  }
}

Figure 9 ErrorLogger3 Class

  public class ErrorLogger3 {
  const String fname = "c:\\temp\\errlog.txt";
  [FileIOPermission(SecurityAction.Assert,
                    Append=fname)
  ]
  public static void Log(String s) {
    FileStream logStream = new FileStream(fname,
      FileMode.Append, FileAccess.Write);
    StreamWriter logWriter =
      new StreamWriter(logStream);
    logWriter.Write(s);
    logWriter.Close();
    logStream.Close();
  }
}

Figure 10 SecurityAction Enumeration

Permission	When	Target	Notes
LinkDemand	JIT time	class, method	check immediate caller
InheritanceDemand	load time	class, method	check subclasses
Demand	load time	class, method	check all callers
Assert	runtime	class, method
Deny	runtime	class, method
PermitOnly	runtime	class, method
RequestMinimum	grant time	assembly	need these to run
RequestOptional	grant time	assembly	would like these
RequestRefuse	grant time	assembly	don't want these

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