How to: Restrict UDF Code Access Security Permissions

If you do not want a particular user-defined function (UDF) assembly to run with full trust, you must explicitly restrict code access security permissions for it. You can configure code groups and restrict permissions by using the .NET Framework 2.0 Configuration tool.

For example, imagine a scenario where you have a UDF assembly that contains multiple methods. One of the UDF methods performs a custom calculation, and another UDF method in the same assembly calls a Web service to obtain stock quotes. Because your users only use Excel workbooks that call the first (calculation) method, you might want to disable the assembly from having Web access, for increased security.

You have the UDF assembly installed in a folder on the server at C:\UdfAssemblies\CalcAndWebAccessUdf.dll. Because the assembly is on the same computer as Microsoft Office SharePoint Server 2007, when Excel Calculation Services loads the UDF assembly, it is loaded in the MyComputer zone. By default, the MyComputer zone is fully trusted. This means that the UDF assembly is granted full trust permission.

To lock down the UDF assembly so that it cannot have Web access, you must explicitly restrict the permission set that it is granted by following these steps:

  1. Create a new URL-based code group under My_Computer_Zone at the Machine level. Scope the code group to that specific assembly and create a custom permission set.

  2. Configure the custom code group properties so that your policy level has only the permissions from the permission set that is associated with the custom code group. When Excel Calculation Services loads a UDF assembly that resides on the same computer, the assembly is loaded in the MyComputer zone. This means that by default, the UDF assembly is granted full trust. When the custom permission set intersects with the full trust permission set, the result is full trust. To make it so that the a policy has only the permission from the permission set that is associated with your custom code group, you must enable the This policy level will only have the permissions from the permission set associated with this code group property.

For more information about configuring code groups, see the following articles on MSDN:

To create a new code group

  1. Click Start, point to All Programs, point to Administrative Tools, and then click Microsoft .NET Framework 2.0 Configuration.

    This starts the .NET 2.0 Framework Configuration tool.

  2. In the left pane, expand the My Computer node, and then expand the Runtime Security Policy node.

  3. Expand the Machine node.

  4. Expand the Code Groups node.

  5. Expand the All_Code node.

  6. Expand the My_Computer_Zone node.Right-click My_Computer_Zone and then select New to display the Identify the new Code Group dialog box.

  7. Select Create a new code group.

  8. In the Name field, type a name for the new code group, for example, RestrictWebAccessUdf.

  9. Click Next.

  10. To scope the code group to your specific UDF assembly, select URL from the Choose the condition type for this code group.

    This displays the URL field.

  11. In the URL field, type the path to the UDF assembly for which you want to restrict access to the Web, for example, C:\UdfAssemblies\CalcAndWebAccessUdf.dll.

  12. Click Next.

  13. Select Create a new permission set, and then click Next.

  14. In the Name field, type a name for your permission set, for example, AssemblyExecutionCustomPermissionSet.

  15. Click Next.

  16. To give your UDF assembly "assembly execution" permission, select Security from the Assembly Permissions list, and then click Add.

    This displays the Permission Settings dialog box.

  17. Select assemblies the following security permissions.

  18. Select Enable assembly execution.

  19. Click OK, and then click Next.

  20. Click Finish.

    You should see your new custom code group under the My_Computer_Zone node (in this example, RestrictWebAccessUdf).

To make sure that the permission sets are executed

  1. Under the My_Computer_Zone node, right-click the new custom code group (in this example, RestrictWebAccessUdf), and then select Properties.

  2. On the General tab, select the This policy level will only have the permissions from the permission set associated with this code group check box.

  3. Click Apply, and then click OK.

    NoteNote:

    If the UDF method throws an exception because it cannot make the Web service call, you should receive a #VALUE! error in the Excel formula that called the UDF.

    NoteNote:

    If you want to enable Web access for your UDF assembly for testing, you must add the appropriate permission to your custom permission set. To do this, in Step 11 of the "To create a new code group" procedure, select Web Access.

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