Month: March 2016

If you’re in the SharePoint community, and you’ve been living under a rock since August 2015, you may not have heard that Excel Services is no longer present in SharePoint 2016. This is not a as big a deal as it initially sounds, because all (OK.. most) of the features available to Excel Services have been moved over to Office Online server, streamlining the architecture. It does however lead to many changes, and this is certainly true for external data access and authentication. This article hopes to answer the question of how these changes impact data access and the Excel Services Authentication Settings.

My previous article “Adding Excel Services Capabilities to a SharePoint 2016 Farm” dealt with getting the farm enabled with Office Online Server and Excel Services capabilities. This article drills down into using Excel workbooks connected to back end data sources. It does not cover PowerPivot enabled workbooks, or workbooks with an embedded data model – that is the topic of the next article in this series.

The Excel Services authentication settings are use when an Excel workbook is opened in a browser, and the workbook connects directly to data on another server. Note that it does NOT pertain to PowerPivot enabled workbooks (at least that portion of those workbooks that uses the data model) as they utilize a different mechanism entirely.

The settings are accessed from the data tab in Excel. First, click on “Connections” to view the available connections in the workbook. Next select the connection and click “Properties”. Select the “Definition” tab, and at the bottom of the dialog box, click on “Authentication Settings”. It’s a well hidden option.

Before continuing, it should be mentioned that these settings affect Windows credentials only. If the data source uses SQL Authentication, or something proprietary, the credentials will be stored in the Secure Store Service, or embedded in the connection string, and they will be used as proxy accounts – fine grained permissions on the destination server will not be possible.

The issue with Windows credentials is the standard “double hop” problem encountered in Windows authentication. This problem existed in prior versions of Excel Services, with SharePoint being the middle tier server. Office Online Server (OOS) must now reside on a separate server, and this add an extra “hop”, exacerbating the problem. In a nutshell, the problem is how to allow the server (in this case, OOS) to provide an identity to the source server. These settings allow for a number of solutions to this problem.

The two major data sources that support Windows authentication are SQL Server, and SQL Server Analysis Services (SSAS), and they are unique enough to be considered separately. Any other data source that supports Windows authentication can be treated in the same manner as SQL Server.

1. Use the authenticated user’s account

This is the option that you’ll choose if you want the identity of the person opening the workbook to be passed back to the source server when the data is refreshed, or in the case off SSAS, interacted with. It’s also the default option. If you never knew about the Excel Services Option Authentication Options button, this would be the one that you’d use.

Before you go rushing to declare that this is the best option, keep in mind that it means that the user permissions will need to be maintained on the data source for all users accessing it. Sometimes proxy credentials are a better choice. In order to get either option working however, additional configuration will be required.

SQL Server Analysis Services Data Sources

If your data source is SSAS you have two options. You can set up Kerberos Constrained Delegation (KCD) between the OOS Server and the SSAS Server, or you can use EffectiveUserName. Given that Kerberos will be discussed in the SQL Server section, and that my recommendation is to use EffectiveUserName whenever possible, we will cover EffectiveUserName here.

To begin with, EffectiveUserName is turned off by default in OOS. In Excel Services this could be turned on through the user interface in the Excel Services Service Application, but with OOS, this is done through PowerShell. As with the other OOS parameters, it’s a single line on the OOS Server:

Set-OfficeWebAppsFarm -ExcelUseEffectiveUserName:$true

With EffectiveUserName,  a connection is made between the OOS Server and the SSAS server using the machine account of the OOS Server itself. The OOS server has the identity, but not the password or token of the requesting user, and that is provided to the SSAS Server to use when executing queries. This impersonation process requires a high level of trust, and the machine account of the OOS server must be an administrator in the destination SSAS instance.

Adding this account requires the use of SQL Server Management Studio 2016 (SSMS), in prior versions it was not possible to add machine accounts to SSAS. To add it, connect SSMS to the destination SSAS instance, then right click on the server node and select “Properties”. Once the dialog options, select the “Security” node, and click the “Add” button.

Next (and order is important here), select the “Locations” button, and choose “Entire Directory” (or a container where your server accounts live). Click “Ok” and and then click the “Object Types” button. Computer accounts are excluded by default, and this is where you select them.

Once selected, you can enter the machine account in the form of DOMAIN\MachineName. After clicking OK, it should appear in the administrators list with a “$” appended to it. In our case, the machine name is NAUTILUS2016OOS and the domain is NAUTILUS. THis needs to be done for each OOS server in your farm.

At this point, any user interacting with the workbook in a browser will interact with the SSAS server using their own identity, which of course means that the SSAS server needs to know who they are. The end users will need to be granted access to the data sources in SSAS, in the same way they would if they were connecting to it in an Excel client.

SQL Server Data Sources

SQL Server has an equivalent feature to EffectiveUserName, the SetUser command. This feature has been supported for years in SQL Server Reporting Services, but unfortunately was never supported by Excel Services. Unfortunately, it’s still not supported by OOS. SharePoint and OOS both support claims based identities, but SQL server does not and this leaves us with only Kerberos Constrained Delegation to provide the user’s identity to the source system.

In order for this to work, Kerberos must be configured for the OOS Server(s) to trust the account running SQL Server on the destination server to delegate credentials. However, KCD delegates Windows credentials, and as mentioned above, both SharePoint 2016 and OOS use claims based authentication. It is necessary to first retrieve the user’s Windows token in order to delegate it, and this needs to happen on the OOS server, as it is now the final “hop” before connecting to the source system.

In the past, SharePoint provided this capability through the Claims to Windows Token Service (C2WTS). The was perfect for Excel Services, as it ran on top of SharePoint and could leverage this service. With OOS that is no longer an option because it runs on a separate machine, but C2WTS can be made available here as a part of the Windows Identity Foundation (WIF).

WIF is not a prerequisite for OOS, so it must be added first. In order to do so,  launch the Add Roles and Features wizard on each OOS server, proceed to the features section, and select “Windows Identity Foundation”. Allow the wizard to complete.

(Note – if you will be supporting ADFS, you will also need to install the Microsoft Identity Extensions.)

Next, the C2WTS needs to be configured to allow access for the Network Service account. To do so, edit the file “C:\Program Files\Windows Identity Foundation\v3.5\c2wtshost.exe.config” and remove the comments in the <allowedCallers> section for Network Service. When complete, it should appear as below:

<allowedCallers>
<clear/>
<add value=”NT AUTHORITY\Network Service” />
<!– <add value=”NT AUTHORITY\Local Service” /> –>
<!– <add value=”NT AUTHORITY\System” /> –>
<!– <add value=”NT AUTHORITY\Authenticated Users” /> –>
</allowedCallers>

Once edited, run the following Powershell to start the service and set it to automatic start:

Set-Service -Name C2WTS -startuptype “automatic”
Start-Service -Name C2WTS

At this point, we are ready to start delegating credentials.

The following Kerberos settings need to be performed on a directory server, or a server that can run the SetSPN command and the Active Directory Users And Computers tool.

The first step is to add the SQL Server service and its associated account as a Service Principal Name (SPN). We need to do this for both the machine name of the server and its Fully Qualified Domain Name(s). This can be done from either PowerShell or a command prompt.

SetSPN –S MSSQLSvc/NAUTILUS2016SQL.nautilus.local :1433 NAUTILUS\SQLService
SetSPN –S MSSQLSvc/NAUTILUS2016SQL :1433 NAUTILUS\SQLService

In this example, NAUTILUS2016SQL is the name of the SQL server, nautilus.local is its domain, and NAUTILUS\SQLService is the account that SQL service is running as.

Once the SPNs have been added,  open the Active Directory Users and Computers tool. Navigate to the Computers node, select the OOS Server(s), right click and select properties. Click the delegation tab, Select “Trust this computer for delegation to specified servers only” (this is the constrained part…), and “Use any authentication protocol”. Next, click the “Add” button, and then “Users and computers”. Find the SQL service account (in our case, NAUTILUS\SQLService), and save it. You should see its corresponding service appear in the dialog box.

Click OK, save everything and that’s it. KCD should be set up to allow delegation from OOS to SQL server, and any direct connected Excel workbooks should work.

This process will need to be repeated for any data source that supports Windows Authentication.

2. Use a Stored Account

A stored account is a proxy account, which is to say that all users using the workbook in a browser will connect to the data using the same credential – the one that is stored. This option uses the Secure Store Service in SharePoint, which is therefore a requirement. Usage is relatively straightforward. To set up a new Stored Account, navigate to the Manage Service Applications section in Central Admin on the SharePoint farm, and select the Secure Store Service.

Once in the service, Select the “New” button in the ribbon to create a new “Target Application” (Stored Account). Enter an ID, Display Name, and contact email (it’s required for some reason). Then, ensure that the Target Application Type is “Group”.

Once complete, click the “Next” button.

The next screen prompts for the type of credentials, and the default is for Windows credentials, which is what we’re using. Click the “Next” button.

The administrators are those users that can modify the target application (stored account) and set the credentials for it. Add administrators as appropriate. The members are those users that can use the stored account, so this should be set to anyone that can access the workbook.

Now, in addition to any regular users, there is one or more accountsthat MUST be in the members list –  the computer accounts for all of the OOS Servers. This makes sense as the Office Online service itself runs as the Network Account, and it needs to consume the credentials from the SSS – it needs permissions to do so. The account should be entered in the form DOMAIN\MachineName$ – so in our example it’s NAUTILUS\Nautilus2016OOS$.

Finally, click OK to save it. The last step is to set the credentials. Select the target application, and click on the “Set Credentials” button in the ribbon. Enter the account that will be used as the proxy here. This will be the stored account that gets used to access the source data, so it obviously should have access to that data.

If your network is http based, there is one further configuration task to perform on the OOS server. By default, OOS requires https to access the secure store service. The option for this is in a PowerShell parameter, and you can change it with the following PowerShell script:

Set-OfficeWebAppsFarm -AllowHttpSecureStoreConnections:$true

Now that this is complete, go to the Excel Services Authentication dialog in an Excel workbook, select “Use a Stored Account” and enter the name of the Secure Store Service Target application that was created (in the example above, this is SSSExcelTest). Save the workbook to SharePoint, open it in a browser and refresh data. All should work.

This procedure is the same for both Analysis Services and for SQL Server. This approach does not require Kerberos, EffectiveUserName, the C2WTS, or S2S trust for it to work. It is likely the easiest option to get up and running quickly.

As with any proxy account, these accounts should only have enough access to  the source data to serve the worksheet(s) they are used for. These should always be least privilege accounts.

3. None

This option was always poorly named. None does not mean no credentials, or anonymous, it means default, or the Unattended Access Account. Both Excel Services and PerformancePoint had the concept of an Unattended Access Account. This account was really just a default entry in the Secure Store Service. It’s useful when the entire organization has access to a certain body of data. Giving the Unattended Access Account access to that data means that nobody needs to configure anything – it should “just work”.

One of the casualties of the move from Excel Services to OOS was the Unattended Access Account. There is no configuration area or PowerShell where it can be set. It would seem that the “None” option cannot be used at all. However, it can, in a very limited way.

Selecting this option will cause the OOS machine account(s) to be used when attempting to access the source data. Therefore, if we grant access in the source data to the machine account, we are treating it like an Unattended Access Account. If this is acceptable from a security standpoint, then it can be used in this manner.

We have already seen how to do this for SSAS, but there is no option in SSMS to add a machine account as a security user in SQL Server, even with SSMS 2016. It is however possible. The way to do this is to use SSMS to open a new query, and run the following T-SQL command:

create login [NAUTILUS\NAUTILUS2016OOS$] from windows

where NAUTILUS\NAUTILUS2016OOS$ is the machine account. The square brackets are required. This will add the machine account as a user in the security section in SQL Server Management Studio.  Once that is done, the account can be added to individual databases in the same manner as any SQL account by navigating to the database Security\Users folder and adding a new user.  From here, select “SQL User with login” as the User type, click the ellipsis beside Login name, enter the machine account and select “Check Names”. Once added click “OK”, and “OK”, and then that account can be used just like any other account.

SSAS cannot use the “None” parameter in this way due to the way that the EffectiveUserName option works. If EffectiveUserName is enabled in OOS, it will simply use the credentials of the logged in user and be indistinguishable from the “Use the authenticated user’s account” option. If it is not enabled, it will try to connect to SSAS as anonymous and fail. The alternative to failing would be to open up access to all data on the server – failure is the best option here.  The bottom line is that the “None” should never be used with SSAS.

Summary

Below is a Summary of the various authentication options that can be used with Excel and OOS, and the requirements to make them work.

Use authenticated user’s account with EffectiveUserName (SSAS Only)

• ExcelUseEffectiveUserName = true on OOS Server
• OOS Computer account added to SSAS Server Administrator list

Use authenticated user’s account with Kerberos (SSAS or SQL Server)

• CTWTS running on OOS Server
• C2WTS configured to allow Network Service on OOS
• KCD delegation from OOS Server to Source Server/account

Use a stored account (SSAS or SQL Server)

• SSS Set up on SharePoint Farm
• SSS Target account(s) created
• OOS Computer account added to Members for Target account(s)
• Target account configured for Least Privilege access to data

None

• OOS Server account configured with Least privilege access to data (SQL Server Only)
• ExcelUseEffectiveUserName = true on OOS Server (SSAS)

Acknowledgements

I wanted to acknowledge the help and guidance I received while putting this together and testing. Both Kay Unkroth and Jason Haak at Microsoft were quick to answer some of the thornier configuration questions, and fellow MVP Trevor Seward was instrumental in getting me over the Kerberos hump. Thanks guys.

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