Using EffectiveUserName To Impersonate in SSAS

It's common to implement your role-based security in your Analysis Services cube. That way Fred can only see the sales from the east region, but Frank can only see sales from the west region. Role-based security requires connections to your cube identify the end user so the cube can secure itself. One way of tackling this is to implement Kerberos so that the credentials of the end user pass from the end user's laptop, to the report server, to the cube.

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But what if your end user's laptop is on another company's domain? Or what if your end user's laptop is on a home workgroup? That laptop can't start a Kerberos ticket that will be valid on your domain. One option is to throw ISA Server 2006 into this mix, use it to present the user with a login form over HTTPS, then let ISA start the Kerberos ticket:

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For either of the above diagrams, you would setup your data source in SSRS as follows:

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We would definitely recommend the above approach, but we realize that ISA and Kerberos add a good deal of complexity to your infrastructure. So the point of this blog post is to mention a good trick for getting role-based security without requiring your end-users' laptop to be able to start a Kerberos ticket.

Before I mention the main trick that's the focus of this post, I should mention that enabling basic auth over HTTPS in SSRS is one possibility to solve this problem. It's not as good of a user experience as the single sign-on login form you can customize with ISA Server, but it works.


A Checkbox in SSRS Many People Misunderstand

The following setup of a data source in SSRS will allow you to pass the end user's credentials to the cube without having a Kerberos ticket start at the end user's laptop, and without having ISA Server.

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Because you stored credentials, SSRS will open the connection to SSAS using those domain\SSASAdminUser credentials. Because you checked the "Impersonate the authenticated user after a connection has been made to the data source" checkbox, behind the scenes SSRS is going to append the EffectiveUserName property into the connection string, specifying the end user who is running the report. The EffectiveUserName property tells SSAS to pretend to be the specified user so that the results of MDX queries use the role-based security setup for that end user.

Only SSAS administrators can use the EffectiveUserName property, so the domain\SSASAdminUser must be an SSAS admin. The other requirement is that you must setup Kerberos on the SSAS server. To be clear, you need to setup the SPN on the SSAS server and you need to setup the AD configurations for delegation, but you don't need to setup Kerberos on any other servers besides the SSAS server. So this is a fairly quick way to get up in running without having your whole network setup for Kerberos.

Since you're using a service account which is an SSAS admin, carefully validate that you've setup the data source correctly to use EffectiveUserName impersonation. You wouldn't want to show your users more data than they're allowed to see just because you forgot to check a box!


SSRS Subscriptions

What happens when an end user tries to create a new subscription on a report with the above data source setup? You get this annoying error message: "Subscriptions cannot be created because the credentials used to run the report are not stored, or if a linked report, the link is no longer valid." It says the credentials have to be stored. Uh. They are. But it's just complaining that we've checked the "impersonate the authenticated user" checkbox. My personal opinion is that this is an oversight. Feel free to vote for this Connect suggestion.

So we have to go for a workaround. The best workaround I could find is to use an expression-based connection string. So edit your report in BIDS, edit the data source, click the fx (expression) button, and enter the following expression-based connection string:

="Data Source=SSAS;Initial Catalog=YourDB;EffectiveUserName=" & User!UserID

After you deploy it, check the data source has stored credentials. Don't check the "Impersonate the authenticated user" checkbox.

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Now users can create their own subscriptions. When the subscriptions run, the user receives report data according to his role-based security.


Data-Driven Subscriptions

Now with your expression-based connection string, you try to create a data-driven subscription. And you get this error message: "Data-driven subscriptions cannot be created because the credentials used to run the report are not stored, the report is using user-defined parameter values, or if a linked report, the link is no longer valid." I believe it's complaining about the user of User!UserID.

You need to create a separate report for data-driven subscriptions. The expression-based connection string in the report needs to be changed to:

="Data Source=SSAS;Initial Catalog=YourDB;EffectiveUserName=" & Parameters!User.Value

Then you need to create a hidden parameter called User. Then the data-driven subscription needs to pass in the appropriate domain\username into that parameter.

Make sure end users don't have access to this report designed for data-driven subscriptions because if they figure out how to set the User parameter, they could run the report and see someone else's data.


The Dark Side of Expression-Based Connection Strings

Note expression-based connection strings can't be used in shared data sources. So you have to manage this one report at a time. Plus you can't even set the expression-based connection string from Report Manager. So you have to set the expression-based connection string in the report at development time, and this makes it a pain to deploy the same report to dev and prod without changing the RDL. Also, once you have changed your report to use an expression-based connection string, you can't open the query designer. So expression-based connection strings really get in the way of the proper development, deployment, and manageability experience.

So you might consider developing your reports with regular shared data sources. Then after you deploy the reports to the server, you can run .NET code like the RSExpressionBasedConnectionString sample code I worked up. (You'll need to request a login to download that sample code.) Once you've got the sample code, edit the app.config pointing it at a folder in SSRS. It loops through all the reports in that folder, finds SSAS data sources, and changes them all to expression-based connection strings. You'll obviously need to tweak the code a good deal to have it do exactly what you want in your environment, but hopefully that's a start. I might know a few good consultants if you happen to need help customizing this for your needs. :-)

Teo Lachev also suggested to me that you could use an RDCE to accomplish the same thing as that .NET code above. But just like with the .NET code above which has to run after the report has been deployed, you'd have to set each report to use the RDCE after it is deployed. Neither option is ideal, but both would work.


Conclusion

Knowing about that "Impersonate the authenticated user" checkbox is a handy trick to have in your bag. Knowing that it's using EffectiveUserName under the covers is very helpful because it lets you use an expression-based connection string to enable subscriptions. But expression-based connection strings are almost more pain than they are worth. So you might consider automating the setup of those expression-based connection strings with some code like what I have mentioned above.

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