WPF 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 WPF 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 computer. 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:
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
Site of Origin Access
IsolatedStorage (512KB Limit)
Input Method Editors (IMEs)
Tablet Stylus and Ink
Simulated Drag/Drop using Mouse Capture and Move Events
XAML Deserialization (via XamlReader.Load)
|Web Integration||Browser Download Dialog|
Top-Level User-Initiated Navigation
Uniform Resource Identifier Parameters
WPF Content Hosted in an IFRAME
Hosting of Same-Site HTML Pages using Frame
Hosting of Same Site HTML Pages using WebBrowser
Web Services (ASMX)
Web Services (using Windows Communication Foundation)
Document Object Model
|Visuals||2D and 3D|
Media (Site Of Origin and Cross-Domain)
Embedded & System Fonts
CFF & TrueType Fonts
Plaintext and Ink Clipboard Support
Copying Selected Content
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. Additionally, the following features have more detailed information regarding partial trust execution, including special considerations.
XAML (see XAML Overview (WPF)).
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).
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
|General||Window (Application Defined Windows and Dialog Boxes)|
Drag and Drop
XAML Serialization (via XamlWriter.Save)
Source Window Access (HwndHost)
Full Speech Support
Windows Forms Interoperability
|Editing||Rich Text Format Clipboard|
Full XAML support
For XBAP applications, code that exceeds the default permission set will have different behavior depending on the security zone. In some cases, the user will receive a warning when they attempt to install it. The user can choose to continue or cancel the installation. The following table describes the behavior of the application for each security zone and what you have to do for the application to receive full trust.
|Security Zone||Behavior||Getting Full Trust|
|Local computer||Automatic full trust||No action is needed.|
|Intranet and trusted sites||Prompt for full trust||Sign the XBAP with a certificate so that the user sees the source in the prompt.|
|Internet||Fails with "Trust Not Granted"||Sign the XBAP with a certificate.|
The behavior described in the previous table is for full trust XBAPs that do not follow the ClickOnce Trusted Deployment model.
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.
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 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:
Imports System.IO ' File, FileStream, StreamWriter Imports System.IO.IsolatedStorage ' IsolatedStorageFile Imports System.Security ' CodeAccesPermission, IsolatedStorageFileStream Imports System.Security.Permissions ' FileIOPermission, FileIOPermissionAccess Imports System.Windows ' MessageBox Namespace SDKSample Public Class FileHandling Public Sub Save() If IsPermissionGranted(New FileIOPermission(FileIOPermissionAccess.Write, "c:\newfile.txt")) Then ' Write to local disk Using stream As FileStream = File.Create("c:\newfile.txt") Using writer As New StreamWriter(stream) writer.WriteLine("I can write to local disk.") End Using End Using Else MessageBox.Show("I can't write to local disk.") End If End Sub ' Detect whether or not this application has the requested permission Private Function IsPermissionGranted(ByVal requestedPermission As CodeAccessPermission) As Boolean Try ' Try and get this permission requestedPermission.Demand() Return True Catch Return False End Try End Function
If an application does not 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
false as appropriate.
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 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.
Imports System.IO ' File, FileStream, StreamWriter Imports System.IO.IsolatedStorage ' IsolatedStorageFile Imports System.Security ' CodeAccesPermission Imports System.Security.Permissions ' FileIOPermission, FileIOPermissionAccess Imports System.Windows ' MessageBox Namespace SDKSample Public Class FileHandlingGraceful Public Sub Save() If IsPermissionGranted(New FileIOPermission(FileIOPermissionAccess.Write, "c:\newfile.txt")) Then ' Write to local disk Using stream As FileStream = File.Create("c:\newfile.txt") Using writer As New StreamWriter(stream) writer.WriteLine("I can write to local disk.") End Using End Using Else ' Persist application-scope property to ' isolated storage Dim storage As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication() Using stream As New IsolatedStorageFileStream("newfile.txt", FileMode.Create, storage) Using writer As New StreamWriter(stream) writer.WriteLine("I can write to Isolated Storage") End Using End Using End If End Sub ' Detect whether or not this application has the requested permission Private Function IsPermissionGranted(ByVal requestedPermission As CodeAccessPermission) As Boolean Try ' Try and get this permission requestedPermission.Demand() Return True Catch Return False End Try End Function
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 (for more information, see Security and WPF Security Strategy - Platform Security).
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 the BrowserInteropHelper.IsBrowserHosted property, which returns true for XAML browser applications (XBAPs).
IsBrowserHosted only distinguishes whether an application is running in a browser, not which set of permissions an application is running with.
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
|DNS||Access DNS servers||Yes||No|
|Isolated Storage||Assembly isolation by user||Yes||No|
|Isolated Storage||Unknown isolation||Yes||Yes|
|Isolated Storage||Unlimited user quota||Yes||No|
|Media||Safe audio, video, and images||Yes||Yes|
|Security||Managed code execution||Yes||Yes|
|Security||Assert granted permissions||Yes||No|
|User Interface||Safe top level windows||Yes||Yes|
|User Interface||Own Clipboard||Yes||Yes|
|Web Browser||Safe frame navigation to HTML||Yes||Yes|
Cut and Paste is only allowed in partial trust when user initiated.
If you need to increase permissions, you need to change the project settings and the ClickOnce application manifest. For more information, see WPF XAML Browser Applications Overview. The following documents may also be helpful.
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 computer, the intranet, or from a URL that is listed in the browser's trusted or allowed sites. If the application is installed from the intranet or a trusted site, the user will receive the standard ClickOnce prompt notifying them of the elevated permissions. The user can choose to continue or cancel the installation.