Code Access Security and ADO.NET

Artikel
11/16/2012

The .NET Framework offers role-based security as well as code access security (CAS), both of which are implemented using a common infrastructure supplied by the common language runtime (CLR). In the world of unmanaged code, most applications execute with the permissions of the user or principal. As a result, computer systems can be damaged and private data compromised when malicious or error-filled software is run by a user with elevated privileges.

By contrast, managed code executing in the .NET Framework includes code access security, which applies to code alone. Whether the code is allowed to run or not depends on the code's origin or other aspects of the code's identity, not solely the identity of the principal. This reduces the likelihood that managed code can be misused.

Code Access Permissions

When code is executed, it presents evidence that is evaluated by the CLR security system. Typically, this evidence comprises the origin of the code including URL, site, and zone, and digital signatures that ensure the identity of the assembly.

The CLR allows code to perform only those operations that the code has permission to perform. Code can request permissions, and those requests are honored based on the security policy set by an administrator.

Note

Code executing in the CLR cannot grant permissions to itself. For example, code can request and be granted fewer permissions than a security policy allows, but it will never be granted more permissions. When granting permissions, start with no permissions at all and then add the narrowest permissions for the particular task being performed. Starting with all permissions and then denying individual ones leads to insecure applications that may contain unintentional security holes from granting more permissions than required. For more information, see Configuring Security Policy and Security Policy Management.

There are three types of code access permissions:

Code access permissions derive from the CodeAccessPermission class. Permissions are required in order to access protected resources, such as files and environment variables, and to perform protected operations, such as accessing unmanaged code.
Identity permissions represent characteristics that identify an assembly. Permissions are granted to an assembly based on evidence, which can include items such as a digital signature or where the code originated. Identity permissions also derive from the CodeAccessPermission base class.
Role-based security permissions are based on whether a principal has a specified identity or is a member of a specified role. The PrincipalPermission class allows both declarative and imperative permission checks against the active principal.

To determine whether code is authorized to access a resource or perform an operation, the runtime's security system traverses the call stack, comparing the granted permissions of each caller to the permission being demanded. If any caller in the call stack does not have the demanded permission, a SecurityException is thrown and access is refused.

Requesting Permissions

The purpose of requesting permissions is to inform the runtime which permissions your application requires in order to run, and to ensure that it receives only the permissions that it actually needs. For example, if your application needs to write data to the local disk, it requires FileIOPermission. If that permission hasn't been granted, the application will fail when it attempts to write to the disk. However, if the application requests FileIOPermission and that permission has not been granted, the application will generate the exception at the outset and will not load.

In a scenario where the application only needs to read data from the disk, you can request that it never be granted any write permissions. In the event of a bug or a malicious attack, your code cannot damage the data on which it operates. For more information, see Requesting Permissions.

Role-Based Security and CAS

Implementing both role-based security and code-accessed security (CAS) enhances overall security for your application. Role-based security can be based on a Windows account or a custom identity, making information about the security principal available to the current thread. In addition, applications are often required to provide access to data or resources based on credentials supplied by the user. Typically, such applications check the role of a user and provide access to resources based on those roles.

Role-based security enables a component to identify current users and their associated roles at run time. This information is then mapped using a CAS policy to determine the set of permissions granted at run time. For a specified application domain, the host can change the default role-based security policy and set a default security principal that represents a user and the roles associated with that user.

The CLR uses permissions to implement its mechanism for enforcing restrictions on managed code. Role-based security permissions provide a mechanism for discovering whether a user (or the agent acting on the user's behalf) has a particular identity or is a member of a specified role. For more information, see Security Permissions.

Depending on the type of application you are building, you should also consider implementing role-based permissions in the database. For more information on role-based security in SQL Server, see SQL Server Security (ADO.NET).

Assemblies

Assemblies form the fundamental unit of deployment, version control, reuse, activation scoping, and security permissions for a .NET Framework application. An assembly provides a collection of types and resources that are built to work together and form a logical unit of functionality. To the CLR, a type does not exist outside the context of an assembly. For more information on creating and deploying assemblies, see Programming with Assemblies.

Strong-naming Assemblies

A strong name, or digital signature, consists of the assembly's identity, which includes its simple text name, version number, and culture information (if provided), plus a public key and a digital signature. The digital signature is generated from an assembly file using the corresponding private key. The assembly file contains the assembly manifest, which contains the names and hashes of all the files that make up the assembly.

Strong naming an assembly gives an application or component a unique identity that other software can use to refer explicitly to it. Strong naming guards assemblies against being spoofed by an assembly that contains hostile code. Strong-naming also ensures versioning consistency among different versions of a component. You must strong name assemblies that will be deployed to the Global Assembly Cache (GAC). For more information, see Creating and Using Strong-Named Assemblies.

Partial Trust in ADO.NET 2.0

In ADO.NET 2.0, the .NET Framework Data Provider for SQL Server, the .NET Framework Data Provider for OLE DB, the .NET Framework Data Provider for ODBC, and the .NET Framework Data Provider for Oracle can now all run in partially trusted environments. In previous releases of the .NET Framework, only System.Data.SqlClient was supported in less than full-trust applications.

At minimum, a partially trusted application using the SQL Server provider must have execution and SqlClientPermission permissions.

Permission Attribute Properties for Partial Trust

For partial trust scenarios, you can use SqlClientPermissionAttribute members to further restrict the capabilities available for the .NET Framework Data Provider for SQL Server.

Note

The .NET Framework Data Provider for SQL Server requires the Full Trust Security permission in order to open a SqlConnection with SQL Debugging enabled in SQL Server 2000. SQL Debugging for SQL Server 2005 does not utilize this class. See SQL Server 2005 Books Online for details.

The following table lists the available SqlClientPermissionAttribute properties and their descriptions:

Permission attribute property	Description
Action	Gets or sets a security action. Inherited from SecurityAttribute.
AllowBlankPassword	Enables or disables the use of a blank password in a connection string. Valid values are true (to enable the use of blank passwords) and false (to disable the use of blank passwords). Inherited from DBDataPermissionAttribute.
ConnectionString	Identifies a permitted connection string. Multiple connection strings can be identified. Note Do not include a user ID or password in your connection string. In this release, you cannot change connection string restrictions using the .NET Framework Configuration Tool. Inherited from DBDataPermissionAttribute.
KeyRestrictions	Identifies connection string parameters that are allowed or disallowed. Connection string parameters are identified in the form <parameter name>=. Multiple parameters can be specified, delimited using a semicolon (;). Note If you do not specify KeyRestrictions, but you set KeyRestrictionBehavior property to AllowOnly or PreventUsage, no additional connection string parameters are allowed. Inherited from DBDataPermissionAttribute.
KeyRestrictionBehavior	Identifies the connection string parameters as the only additional parameters allowed (AllowOnly), or identifies the additional parameters that are not allowed (PreventUsage). AllowOnly is the default. Inherited from DBDataPermissionAttribute.
TypeID	Gets a unique identifier for this attribute when implemented in a derived class. Inherited from Attribute.
Unrestricted	Indicates whether unrestricted permission to the resource is declared. Inherited from SecurityAttribute.

ConnectionString Syntax

The following example demonstrates how to use the connectionStrings element of a configuration file to allow only a specific connection string to be used. See Connection Strings (ADO.NET) for more information on storing and retrieving connection strings from configuration files.

<connectionStrings>
  <add name="DatabaseConnection" 
    connectionString="Data Source=(local);Initial 
    Catalog=Northwind;Integrated Security=true;" />
</connectionStrings>

KeyRestrictions Syntax

The following example enables the same connection string, enables the use of the Encrypt and PacketSize connection string options, but restricts the use of any other connection string options.

<connectionStrings>
  <add name="DatabaseConnection" 
    connectionString="Data Source=(local);Initial 
    Catalog=Northwind;Integrated Security=true;"
    KeyRestrictions="Encrypt=;Packet Size=;"
    KeyRestrictionBehavior="AllowOnly" />
</connectionStrings>

KeyRestrictionBehavior with PreventUsage Syntax

The following example enables the same connection string and allows all other connection parameters except for User Id, Password and Persist Security Info.

<connectionStrings>
  <add name="DatabaseConnection" 
    connectionString="Data Source=(local);Initial 
    Catalog=Northwind;Integrated Security=true;"
    KeyRestrictions="User Id=;Password=;Persist Security Info=;"
    KeyRestrictionBehavior="PreventUsage" />
</connectionStrings>

KeyRestrictionBehavior with AllowOnly Syntax

The following example enables two connection strings that also contain Initial Catalog, Connection Timeout, Encrypt, and Packet Size parameters. All other connection string parameters are restricted.

<connectionStrings>
  <add name="DatabaseConnection" 
    connectionString="Data Source=(local);Initial 
    Catalog=Northwind;Integrated Security=true;"
    KeyRestrictions="Initial Catalog;Connection Timeout=;
       Encrypt=;Packet Size=;" 
    KeyRestrictionBehavior="AllowOnly" />

  <add name="DatabaseConnection2" 
    connectionString="Data Source=SqlServer2;Initial 
    Catalog=Northwind2;Integrated Security=true;"
    KeyRestrictions="Initial Catalog;Connection Timeout=;
       Encrypt=;Packet Size=;" 
    KeyRestrictionBehavior="AllowOnly" />
</connectionStrings>

Enabling Partial Trust with a Custom Permission Set

To enable the use of System.Data.SqlClient permissions for a particular zone, a system administrator must create a custom permission set and set it as the permission set for a particular zone. Default permission sets, such as LocalIntranet, cannot be modified. For example, to include System.Data.SqlClient permissions for code that has a Zone of LocalIntranet, a system administrator could copy the permission set for LocalIntranet, rename it to "CustomLocalIntranet", add the System.Data.SqlClient permissions, import the CustomLocalIntranet permission set using the Code Access Security Policy Tool (Caspol.exe), and set the permission set of LocalIntranet_Zone to CustomLocalIntranet.

Sample Permission Set

The following is a sample permission set for the .NET Framework Data Provider for SQL Server in a partially trusted scenario. For information on creating custom permission sets, see Configuring Permission Sets Using Caspol.exe.

<PermissionSet class="System.Security.NamedPermissionSet"
  version="1"
  Name="CustomLocalIntranet"
  Description="Custom permission set given to applications on
    the local intranet">

<IPermission class="System.Data.SqlClient.SqlClientPermission, System.Data, Version=2.0.0000.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
version="1"
AllowBlankPassword="False">
<add ConnectionString="Data Source=(local);Integrated Security=true;"
 KeyRestrictions="Initial Catalog=;Connection Timeout=;
   Encrypt=;Packet Size=;" 
 KeyRestrictionBehavior="AllowOnly" />
 </IPermission>
</PermissionSet>

Verifying ADO.NET Code Access Using Security Permissions

For partial-trust scenarios, you can require CAS privileges for particular methods in your code by specifying a SqlClientPermissionAttribute. If that privilege is not allowed by the restricted security policy in effect, an exception is thrown before your code is run. For more information on security policy, see Security Policy Management and Security Policy Best Practices.

Example

The following example demonstrates how to write code that requires a particular connection string. It simulates denying unrestricted permissions to System.Data.SqlClient, which a system administrator would implement using a CAS policy in the real world.

Security Note:
When designing CAS permissions for ADO.NET, the correct pattern is to start with the most restrictive case (no permissions at all) and then add the specific permissions that are needed for the particular task that the code needs to perform. The opposite pattern, starting with all permissions and then denying a specific permission, is not secure because there are many ways of expressing the same connection string. For example, if you start with all permissions and then attempt to deny the use of the connection string "server=someserver", the string "server=someserver.mycompany.com" would still be allowed. By always starting by granting no permissions at all, you reduce the chances that there are holes in the permission set.

When designing CAS permissions for ADO.NET, the correct pattern is to start with the most restrictive case (no permissions at all) and then add the specific permissions that are needed for the particular task that the code needs to perform. The opposite pattern, starting with all permissions and then denying a specific permission, is not secure because there are many ways of expressing the same connection string. For example, if you start with all permissions and then attempt to deny the use of the connection string "server=someserver", the string "server=someserver.mycompany.com" would still be allowed. By always starting by granting no permissions at all, you reduce the chances that there are holes in the permission set.

The following code demonstrates how SqlClient performs the security demand, which throws a SecurityException if the appropriate CAS permissions are not in place. The SecurityException output is displayed in the console window.

Private Sub TestCAS(ByVal connectString1 As String, ByVal connectString2 As String)
    ' Simulate removing SqlClient permissions. 
    Dim permission As New SqlClientPermission(PermissionState.Unrestricted)
    permission.Deny()

    ' Try to open a connection. 
    Try 
        Using connection As New SqlConnection(connectString1)
            connection.Open()
            Console.WriteLine("Connection opened, unexpected.")
        End Using 

    Catch ex As System.Security.SecurityException
        Console.WriteLine("Failed, as expected: {0}", _
            ex.FirstPermissionThatFailed)

        ' Uncomment the following line to see Exception details. 
        ' Console.WriteLine("BaseException: {0}", ex.GetBaseException()) 
    End Try

    SqlClientPermission.RevertAll()

    ' Add permission for a specific connection string. 
    ' This would typically be achieved by the administrator   
    ' deploying a CAS policy, not in your code.
    permission = New SqlClientPermission(PermissionState.None)
    permission.Add(connectString1, "", KeyRestrictionBehavior.AllowOnly)

    permission.PermitOnly()

    ' Try again, it should succeed now. 
    Try 
        Using connection As New SqlConnection(connectString1)
            connection.Open()
            Console.WriteLine("Connection opened, as expected.")
        End Using 

    Catch ex As System.Security.SecurityException
        Console.WriteLine("Unexpected failure: {0}", ex.Message)
    End Try 

    ' Try a different connection string. This should fail. 
    Try 
        Using connection As New SqlConnection(connectString2)
            connection.Open()
            Console.WriteLine("Connection opened, unexpected.")
        End Using 

    Catch ex As System.Security.SecurityException
        Console.WriteLine("Failed, as expected: {0}", ex.Message)
    End Try 
End Sub

static void TestCAS(string connectString1, string connectString2)
{
    // Simulate removing SqlClient permissions.
    SqlClientPermission permission = new 
        SqlClientPermission(PermissionState.Unrestricted);
    permission.Deny();

    // Try to open a connection. 
    try
    {
        using (SqlConnection connection = new 
                   SqlConnection(connectString1))
        {
            connection.Open();
            Console.WriteLine("Connection opened, unexpected.");
        }
    }
    catch (System.Security.SecurityException ex)
    {
        Console.WriteLine("Failed, as expected: {0}", 
            ex.FirstPermissionThatFailed);

        // Uncomment the following line to see Exception details. 
        //Console.WriteLine("BaseException: " + ex.GetBaseException());
    }

    SqlClientPermission.RevertAll();

    // Add permission for a specific connection string. 
    // This would typically be achieved by the administrator deploying  
    // a CAS policy, not in your code.
    permission = new SqlClientPermission(PermissionState.None);
    permission.Add(connectString1,"", KeyRestrictionBehavior.AllowOnly);

    permission.PermitOnly();

    // Try again, it should succeed now. 
    try
    {
        using (SqlConnection connection = new SqlConnection(connectString1))
        {
            connection.Open();
        }
        Console.WriteLine("Connection opened, as expected.");
    }
    catch (System.Security.SecurityException ex)
    {
        Console.WriteLine("Unexpected failure: {0}", ex.Message);
    }

    // Try a different connection string. This should fail. 
    try
    {
        using (SqlConnection connection = new SqlConnection(connectString2))
        {
            connection.Open();
        }
        Console.WriteLine("Connection opened, unexpected.");
    }
    catch (System.Security.SecurityException ex)
    {
        Console.WriteLine("Failed, as expected: {0}", ex.Message);
    }
}

You should see this output in the Console window:

Failed, as expected: <IPermission class="System.Data.SqlClient.
SqlClientPermission, System.Data, Version=2.0.0.0, 
  Culture=neutral, PublicKeyToken=b77a5c561934e089" version="1"
  AllowBlankPassword="False">
<add ConnectionString="Data Source=(local);Initial Catalog=
  Northwind;Integrated Security=SSPI" KeyRestrictions=""
KeyRestrictionBehavior="AllowOnly"/>
</IPermission>

Connection opened, as expected.
Failed, as expected: Request failed.

Interoperability with Unmanaged Code

Code that runs outside the CLR is called unmanaged code. Therefore, security mechanisms such as CAS cannot be applied to unmanaged code. COM components, ActiveX interfaces, and Win32 API functions are examples of unmanaged code. Special security considerations apply when executing unmanaged code so that you do not jeopardize overall application security. For more information, see Interoperating with Unmanaged Code.

The .NET Framework also supports backward compatibility to existing COM components by providing access through COM interop. You can incorporate COM components into a .NET Framework application by using COM interop tools to import the relevant COM types. Once imported, the COM types are ready to use. COM interop also enables COM clients to access managed code by exporting assembly metadata to a type library and registering the managed component as a COM component. For more information, see Advanced COM Interoperability.

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