Windows Presentation Foundation Partial Trust Security

In general, Internet applications should be restricted from having direct access to critical system resources, to prevent malicious damage. By default, HTML and client-side scripting languages are not able to access critical system resources. Because Windows Presentation Foundation (WPF) browser-hosted applications can be launched from the browser, they should conform to a similar set of restrictions. To enforce these restrictions, WPF relies on both Code Access Security (CAS) and ClickOnce (see Windows Presentation Foundation Security Strategy - Platform Security). By default, browser-hosted applications request the Internet zone CAS set of permissions, irrespective of whether they are launched from the Internet, the local intranet, or the local machine. Applications that run with anything less than the full set of permissions are said to be running with partial trust.

WPF provides a wide variety of support to ensure that as much functionality as possible can be used safely in partial trust, and along with CAS, provides additional support for partial trust programming.

This topic contains the following sections.

• WPF Feature Partial Trust Support
• Partial Trust Programming
• Managing Permissions
• Related Topics

WPF Feature Partial Trust Support

The following table lists the high level features of Windows Presentation Foundation (WPF) that are safe to use within the limits of the Internet zone permission set.

Table 1: WPF Features that are Safe in Partial Trust

This table covers the WPF features at a high level. For more detailed information, the Windows Software Development Kit (SDK) documents the permissions that are required by each member in WPF. The following, however, have

The following WPF features have special considerations for running in partial trust, including:

• XAML (see XAML Overview).

• Popups (see System.Windows.Controls.Primitives.Popup).

• Drag and Drop (see Drag and Drop Overview).

• Clipboard (see System.Windows.Clipboard).

• Imaging (see System.Windows.Controls.Image).

• Serialization (see System.Windows.Markup.XamlReader.Load, System.Windows.Markup.XamlWriter.Save).

• Open File Dialog Box (see Microsoft.Win32.OpenFileDialog).

The following table outlines the WPF features that are not safe to run within the limits of the Internet zone permission set:

Table 2: WPF Features that are Not Safe in Partial Trust

Partial Trust Programming

For XBAP applications, code that exceeds the default Internet zone permission set will be detected by CAS and will result in a security exception being raised and the application ending. While this protects users, it does not provide for the best user experience.

In general, code that may exceed the allowed permissions is likely to be common code that is shared between both standalone and browser-hosted applications. CAS and WPF offer several techniques for managing this scenario.

Detecting Permissions using CAS

In some situations, it is possible for shared code in library assemblies to be used by both standalone applications and XBAPs. In these cases, code may execute functionality that could require more permissions than the application's awarded permission set allows. Your application can detect whether or not it has a certain permission by using Microsoft .NET Framework version 3.0 security. Specifically, it can test whether it has a specific permission by calling the Demand method on the instance of the desired permission. This is shown in the following example, which has code that queries for whether it has the ability to save a file to the local disk:

public class SharedClass
{
    ...
    public static void Save()
        if( IsPermissionGranted(new FileIOPermission(FileIOPermissionAccess.Write, @"c:\newfile.txt") ) ) {
            // Write to local disk
            using (FileStream stream = File.Create(@"c:\newfile.txt"))
            using (StreamWriter writer = new StreamWriter(stream))
            {
                writer.WriteLine("I can write to local disk.");
            }
        }
        else
        {
            MessageBox.Show("I can't write to local disk.");
        }
    }
    
    // Detect whether or not this application has the requested permission
    bool IsPermissionGranted(CodeAccessPermission requestedPermission)
    {
        try
        {
            // Try and get this permission
            requestedPermission.Demand();
            return true;
        }
        catch
        {
            return false;
        }
    }
}

If an application doesn't have the desired permission, the call to Demand will throw a security exception. Otherwise, the permission has been granted. IsPermissionGranted encapsulates this behavior and returns true or false as appropriate.

Graceful Degradation of Functionality

Being able to detect whether code has the permission to do what it needs to do is interesting for code that can be executed from different zones. While detecting the zone is one thing, it is far better to provide an alternative for the user, if possible. For example, a full trust application typically enables users to create files anywhere they want, while a partial trust application can only create files in isolated storage. If the code to create a file exists in an assembly (.dll) that is shared by both full trust (standalone applications) and partial trust (browser-hosted applications), and both applications want users to be able to create files, the shared code should detect whether it is running in partial or full trust before creating a file in the appropriate location. The following code demonstrates both:

public class SharedClass
{
    public static void Save()
    {
        if( IsPermissionGranted(new FileIOPermission(FileIOPermissionAccess.Write, @"c:\newfile.txt") ) ) {
            // Write to local disk
            using (FileStream stream = File.Create(@"c:\newfile.txt"))
            using (StreamWriter writer = new StreamWriter(stream))
            {
                writer.WriteLine("I can write to local disk.");
            }
        }
        else
        {
            // Persist application-scope property to 
            // isolated storage
            IsolatedStorageFile storage = 
              IsolatedStorageFile.GetUserStoreForApplication();
            using (IsolatedStorageFileStream stream = 
              new IsolatedStorageFileStream(
                "newfile.txt", FileMode.Create, storage))
            using (StreamWriter writer = new StreamWriter(stream))
            {
                writer.WriteLine(
                  "I can write to Isolated Storage");
            }
        }
    }

    // Detect whether or not this application has the requested permission
    bool IsPermissionGranted(CodeAccessPermission requestedPermission)
    {
        try
        {
            // Try and get this permission
            requestedPermission.Demand();
            return true;
        }
        catch
        {
            return false;
        }
    }
}

In many cases, you should be able to find a partial trust alternative.

In a controlled environment, such as an intranet, custom managed frameworks can be installed across the client base into the global assembly cache (GAC). These libraries can execute code that requires full trust, and be referenced from applications that are only allowed partial trust by using AllowPartiallyTrustedCallersAttribute (see Windows Presentation Foundation Security and Windows Presentation Foundation Security Strategy - Platform Security for more information).

Browser Host Detection

Using CAS to check for permissions is a suitable technique when you need to check on a per-permission basis. Although, this technique depends on catching exceptions as a part of normal processing, which is not recommended in general, and can have performance issues. Instead, if your XAML browser application (XBAP) only runs within the Internet zone sandbox, you can use System.Windows.Interop.BrowserInteropHelper.IsBrowserHosted, which returns true for XAML browser applications (XBAPs).

See the Detecting if Browser-Hosted Sample.

Note:
that IsBrowserHosted only distinguishes whether an application is running in a browser, not which set of permissions an application is running with.

Managing Permissions

By default XBAPs run with partial trust (default Internet zone permission set). However, depending on the requirements of the application, it is possible to change the set of permissions from the default. For example, if an XBAPs is launched from a local intranet, it can take advantage of an increased permission set, which is shown in the following table.

Table 3: LocalIntranet and Internet Permissions

If you need to increase permissions, you can use one of the following tools:

• Manifest Generation and Editing Tool (Mage.exe).

• Manifest Generation and Editing Tool, Graphical Client (MageUI.exe).

• Visual Studio ClickOnce Deployment and Security.

If your XBAP requires full trust, you can use the same tools to increase the requested permissions, although an XBAP will only receive full trust if it is installed on and launched from the local machine. This means you do not retain the automatic update support you get when you publish XBAPs to web servers.

See Also

Concepts

Windows Presentation Foundation Security
Windows Presentation Foundation Security Strategy - Platform Security
Windows Presentation Foundation Security Strategy - Security Engineering