Working with Assemblies : Overview of Security Changes in .NET 4.0

Assemblies contain code that is executed when you run the application. As for the operating system and for any development environment, code is executed according to security rules that prevent the code from unauthorized access to system resources. The .NET Framework 4.0 introduces a new security model, highly simplified if compared to the previous Code Access Security platform. The code will still be classified as fully trusted and partially trusted, in which full trust means that the code has elevated permissions for accessing resources, whereas partial trust means that the code is restricted by the permissions it has. The security model provided by the CLR is now easier to understand and to implement, differently from what Code Access Security was in the past.

Note on Security Changes

Extensive changes have been introduced to the security model for code access, whereas the role-based security model basically offers the same features as in the past. For this reason in this section we cover changes about the code access security model, whereas for further details on the role-based model, you can check out the MSDN documentation here: http://msdn.microsoft.com/en-us/library/shz8h065(VS.100).aspx.

The following are the major changes in the security model offered by .NET 4.0:

Code Access Security policies and machine-wide security policies are now turned off by default.

The transparency model has been enforced and applied to the .NET Framework and managed applications. Basically the transparency model can separate code that runs as part of the application (transparent code) and code that runs as part of the .NET infrastructure (critical code). As a result, critical code can access privileged resources, such as native code, whereas transparent code can only access resources allowed by the specified permissions set and cannot invoke or inherit from critical code; with the transparency model groups of code isolated based on privileges. Such privileges are divided into full-trust and partial-trust in the sandboxed model.

The enforcement of the transparency model is also the reason why the .NET Framework configuration tool is no longer available for setting CAS policies.
The sandboxed model allows running code in a restricted environment that grants code the only permissions it actually requires, and the natural place for the model is the application domains described in the previous section.
Desktop applications always run as fully trusted. This is also true for applications started from Windows Explorer, a command prompt, and from a network share.
Permissions are still a central part in security, but some security actions from the System.Security.Permission.SecurityAction class have been deprecated. In detail they are Deny, RequestMinimum, RequestOptional, and RequestRefuse.
You can expose partially trusted assemblies via the AllowPartiallyTrustedCallers attribute.
To enable constraints on types that can be used as evidence objects, .NET Framework 4 introduces the System.Security.Policy.EvidenceBase base class that must be inherited from all objects that want to be candidate as evidence.

Transparency Model

The transparency model is not new in the .NET Framework; it was first introduced with version 2.0 as a mechanism for validating code efficiency. In .NET 4 it has been revisited (this is the reason why it is also known as Level 2) and provides an enforcement mechanism for code separation.

The next sections provide explanations and code examples about new security features in .NET Framework 4 with Visual Basic 2010.

Permissions

With the exceptions described in the previous bulleted list for deprecated security actions, applying permissions in the new security model is similar to the previous versions of the .NET Framework. This means that you can leverage permissions from the System.Security.Permissions namespace, such as FileIOPermission, UIPermission, IsolatedStoragePermission, and EnvironmentPermission. The following code demonstrates how you use the declarative syntax for implementing a class that requires the caller code having the FileIOPermission to execute. Such a class simply implements a method that returns an XDocument from a text file:

'The caller code will need the FileIOPermission permission
'with unrestricted access otherwise it will fail
<FileIOPermission(Security.Permissions.SecurityAction.Demand,
                  Unrestricted:=True)>
Class XmlParse

    Shared Function String2Xml(ByVal fileName As String) As XDocument
        'Expects an Xml-formatted string
        Return XDocument.Parse(fileName)
    End Function
End Class

You can also use the imperative syntax, which looks like this:

Dim fp As New FileIOPermission(PermissionState.Unrestricted)
Try
    fp.Demand()

Catch ex As Security.SecurityException

End Try

You create an instance of the required permission and then invoke Demand for checking if the application has that level of permissions. If not, a System.Security.Security Exception is thrown.

The Transparency Level 2

By default, when you create a new application it relies on security rules provided by the Transparency Level 2 of .NET Framework 4.0. The level name has this form to allow distinction from the old transparency level of previous .NET versions (known as Transparency Level 1). So the Transparency Level 2 security rules are applied implicitly, but a better idea is applying them explicitly by applying the System.Security.SecurityRules attribute that can be added at the assembly level as follows:

<Assembly: SecurityRules(Security.SecurityRuleSet.Level2)>

Applying the attribute explicitly is appropriate for code reading and future maintenance and avoids confusions. This level of enforcement brings into the .NET Framework some new ways of thinking about security policies. Most rely on the concept of host, where this means an environment is responsible for executing applications; ClickOnce, ASP.NET, and Internet Explorer are host examples. For code trust, applications that are not hosted, such as programs launched from a command prompt, from Windows Explorer or from a shared network path, now run as full-trust. Instead, hosted or sandboxed applications still run according to host-based policies and run as partial-trust. For hosted and sandboxed applications, it is worth mentioning that they are considered as transparent because they run with the limited permissions set granted by the sandbox. This means that you will no longer need to check for permissions when running partially trusted code, because transparent applications run with the permissions set granted by the sandbox, so your only preoccupation should be targeting the sandbox permissions set and to not write code requiring the full-trust policy. Talking about transparency, it is important to mention that its mechanism can separate code that is part of the .NET infrastructure (and that thus requires high privileges such as invoking native code), which is called critical code, and code that is part of the application, also known as transparent code. The idea behind the scenes is separating groups of code based on privileges. When working with sandboxes, such privileges are of two types: fully trusted, which is the unrestricted level, and the partially trusted, which is the level restricted to the permission set established in the sandbox.

Desktop Applications

With the Transparency Level 2 enabled, desktop applications run as full-trust.

The System.Security.SecurityRules attribute is not the only one that you can apply for establishing permissions rules. There are other attributes available, summarized in Table 1.

Table 1. Security Attributes
Attribute	Description
SecurityTransparent	Specifies that the code is transparent, meaning that it can be accessed by partially trusted code, that it cannot allow access to protected resources, and that it cannot cause an elevation of privileges. All types and members are transparent.
SecurityCritical	Code introduced by types exposed from the assembly is considered as security-critical meaning that it can perform operations that require an elevation of privileges, whereas all other code is transparent. Methods overridden from abstract classes or implemented via an interface must be also explicitly marked with the attribute.
SecuritySafeCritical	Specifies that types expose critical code but allows access from partially trusted assemblies.

If you do not specify any other attribute other than SecurityRules, for fully trusted assemblies the runtime considers all code as security-critical, thus callable only from fully trusted code, except where this could cause inheritance violations. If the assembly is instead partially trusted, specifying no attribute other than SecurityRules will make the runtime consider types and members as transparent by default but they can be security-critical or security-safe-critical. For further detail on inheritance in the transparency model and on attributes listed in Table 46.1, visit the following page in the MSDN Library: http://msdn.microsoft.com/en-us/library/dd233102(VS.100).aspx . So this is the reason why it is opportune to explicitly provide the most appropriate attribute. The following is an example of applying both the SecurityRules and SecurityTransparent attributes:

<Assembly: SecurityRules(Security.SecurityRuleSet.Level2)>
<Assembly: SecurityTransparent()>
Class Foo
End Class

Tips on SecurityTransparent

Transparency enforcements are handled by the Just-in-Time compiler and not by the CLR infrastructure. This means that if you apply the SecurityTransparent attribute to an assembly, the assembly cannot call transparent and security-safe-critical types and members independently from the permissions set (including full-trust). In such a scenario, if the code attempts to access a security-critical type or member, a MethodAccessException will be thrown.

Sandboxing

You can execute partially trusted code within a sandbox that runs with the specified permissions set. Code, including assemblies, executed within the sandbox will be also granted to just the specified permissions set. To create and run a sandbox, you need an instance of the AppDomain class. The example here creates a sandbox for running an external assembly given the LocalIntranet zone’s permissions. Before showing the sandbox example, follow these steps:

1.	Create a new Console application and name the new project as ExternalApp.
2.	In the Main method simply add a Console.Writeline statement for showing whatever text message you like.
3.	Build the project; then create a new folder named C:\MyApps and copy the newly generated ExternalApp.exe into C:\MyApps.

Such steps are required to have a simple external assembly to run inside the security sandbox. Now close the ExternalApp project and create a new Console project, naming it SandBox. The goal is to create a sandbox with LocalIntranet permission and run an external assembly inside the sandbox so that this external application will also be granted the same permissions. When ready, first add the following Imports directives:

Imports System.Security
Imports System.Security.Policy
Imports System.Reflection

Now move inside the Main method. The first thing you need is an Evidence object that you assign with the required permissions set, as demonstrated by the following code:

Dim ev As New Evidence()
ev.AddHostEvidence(New Zone(SecurityZone.Intranet))

When you have the Evidence instance, you can get a sandbox with the specified permissions as demonstrated by the following line:

Dim permSet As PermissionSet = SecurityManager.GetStandardSandbox(ev)

The SecurityManager.GetStandardSandbox returns a sandbox limited to the specified permissions. This sandbox will be used later when running the external assembly. As an alternative you can set your own permissions creating your custom permissions set using the PermissionSet object as follows:

Dim permSet As New PermissionSet(Permissions.PermissionState.None)
permSet.AddPermission( _
        New SecurityPermission(SecurityPermissionFlag.Execution))
permSet.AddPermission(New UIPermission(PermissionState.Unrestricted))

At this point we can put our hands on application domains. The first thing to do is create an instance of the AppDomainSetup class for specifying the working directory of the external assembly:

Dim ads As New AppDomainSetup()
ads.ApplicationBase = "C:\MyApps"

At this point we just need to set the host Evidence and then create the AppDomain, passing the security information, finally invoking AppDomain.ExecuteAssembly to run the sand-boxed assembly:

Dim hostEvidence As New Evidence()
Dim sandbox As AppDomain = AppDomain.
    CreateDomain("Sandboxed Domain", hostEvidence, ads, permSet, Nothing)

sandbox.ExecuteAssemblyByName("ExternalApp")

The AppDomain.CreateDomain method has an overload that allows creating an application domain with a permissions set. Because the application domain has security permissions, an instance of the Evidence class is required to tell the runtime that the assembly will be affected by such permissions. Other arguments are the AppDomainSetup instance and the permissions set under which the external assembly is going to be run. The last null argument can be replaced with a reference to the strong name, in case you want to add it to the full trust list. This would first require the current application to be signed with a strong name and then by getting a reference to the strong name via the System.Security.Policy.StrongName class as shown in the following line:

Dim fullTrustAssembly As StrongName = Assembly.
    GetExecutingAssembly.Evidence.GetHostEvidence(Of StrongName)()

The Assembly.Evidence.GetHostEvidence(Of StrongName) method returns the reference to the strong name. (The System.Assembly class is discussed in the next chapter on Reflection). Finally, you pass the strong name reference to AppDomain.CreateDomain as follows:

Dim sandbox As AppDomain = AppDomain.
    CreateDomain("Sandboxed Domain", hostEvidence, ads,
                 permSet, fullTrustAssembly)

Listing 1 shows the complete code example for your convenience.

Listing 1. Running a Sandboxed Assembly

Imports System.Security
Imports System.Security.Policy
Imports System.Reflection

Module Module1

    Sub Main()
        Dim ev As New Evidence()
        ev.AddHostEvidence(New Zone(SecurityZone.Intranet))

        Dim permSet As PermissionSet = SecurityManager.GetStandardSandbox(ev)

        Dim ads As New AppDomainSetup()
        ads.ApplicationBase = "C:\MyApps"

        Dim hostEvidence As New Evidence()
        Dim sandbox As AppDomain = AppDomain.
            CreateDomain("Sandboxed Domain", hostEvidence, ads,
                         permSet, Nothing)

        'The assembly runs in a LocalIntranet sandboxed environment
        sandbox.ExecuteAssemblyByName("ExternalApp")
    End Sub
End Module

Sandboxes Complexity

Working with sandboxes can include complex scenarios. Particularly, you might have the need to execute not-trusted code from an external assembly with customized permissions sets. This also requires advanced application domains concepts. Fortunately the MSDN Library provides an interesting walk-through covering these scenarios, available at http://msdn.microsoft.com/en-us/library/bb763046(VS.100).aspx . This is also useful to get a practical example about implementing the MarshalByRefObject for dynamic code execution within application domains.

Conditional APTCA

You can allow an assembly to be called by partially trusted code by applying the System.Security.AllowPartiallyTrustedCallers attribute at the assembly level. This can be accomplished as follows:

Imports System.Security


<Assembly: AllowPartiallyTrustedCallers()>

Without this attribute, only full-trusted code can call the assembly. Different from previous versions, in the .NET Framework 4.0 this attribute no longer requires an assembly to be signed with a strong name, and its presence involves in the security checks all security functions present in the code.

Migrating from Old CAS-Based Code

If you move your existing code to .NET Framework 4 and you made use of Code Access Security policies, you might be advised with a message saying that CAS is obsolete. In these particular situations you can add a specific section to the application configuration file, which allows legacy policies that look like this:

<configuration>
  <runtime>
    <NetFx40_LegacySecurityPolicy enabled="true"/>
  </runtime>
</configuration>

Of course, you always need to check if legacy policies are appropriate in the particular scenario you are facing. The suggestion is to read the MSDN documentation about CAS migration, available at http://msdn.microsoft.com/en-us/library/ee191568(vs.100).aspx.