My syntax highlighter is a bit broken.  Until I fix it, the TSQL shown below will be full of crud.  Fix it before trying it on your system.  Or just wait until I fix up the highlighter.  Your choice, really.

Much has been written about why SQL’s xp_cmdshell is a bad thing.  It has its uses, of course, and sometimes we just have to enable it for our apps to work.

In my most recent post about SQL, you would’ve detected a certain.. disdain.. for applications developers.  It’s not their fault, though.  They’re just trying to write something that works.  In my workplace, there is a “just make it work” mentality, which can lead to some awful misconfigurations.  That’s why this post is dedicated to just how bad xp_cmdshell can be.

By default, xp_cmdshell is disabled, and with good reason.  If it’s not set up just so, it can be exploited.  The gist of it is this:  if you can run a command from a DOS shell, non-interactively, xp_cmdshell will run it, too.  So it stands to reason that you should lock xp_cmdshell down as much as possible.  But this is at odds with the “just make it work” mentality, and sometimes security is compromised as a result.

If you’ve set up your SQL server per best practice, your SQL Server service account is an unprivileged domain account.  That means it’s just a regular account, not a member of any special domain or local groups.  When you tell the SQL installer to use this account, it sets up file permissions, registry permissions and local policies so that your service account has all the things it needs to run SQL properly.  But by default, the SQL installer doesn’t prompt you for a domain account.  No.  Left to its own devices, it will let you choose from a drop-down:

<insert screenshot here that Crayon syntax highlighter doesn’t like 🙁 >

Interesting.  The local system account is here, as well as the option to browse.  Hey, Local System looks good.  So, for this demonstration, let’s assume the naive person installing SQL selects this account.  For those of you not in the know, the Local System account has the keys to the city over the local machine.  It has no rights on any other system.  But for this demo, that doesn’t matter.  Let’s then assume the “Use the same account for all SQL Server services” is clicked.

So we carry on installing, and voila, we have a running SQL instance.  Let’s say the apps developer asks for xp_cmdshell to be enabled.  That’s easy enough to do:

EXEC master.dbo.sp_configure 'show advanced options', 1
RECONFIGURE
go
EXEC master.dbo.sp_configure 'xp_cmdshell', 1
RECONFIGURE
go

And we get a successful result!

Configuration option ‘show advanced options’ changed from 0 to 1. Run the RECONFIGURE statement to install.
Configuration option ‘xp_cmdshell’ changed from 0 to 1. Run the RECONFIGURE statement to install.

Let’s try a command:

xp_cmdshell 'whoami.exe'

Hey, this is great!  It WORKS!!!!!!!!!!!!!!!!!!!!!!

Let’s try another one:

xp_cmdshell 'dir c:'

Yay!  More working xp_cmdshell goodness.

We can even use it to write to a text file:

exec xp_cmdshell 'echo hello > c:\file.txt'
exec xp_cmdshell 'echo appended data >> c:\file.txt'
exec xp_cmdshell 'echo more data >> c:\file.txt'

And the result is, funnily enough, a text file C:\FILE.TXT:

hello
appended data
more data

Let’s try something a little more interesting.

xp_cmdshell 'format c:'

We should all breathe a sigh of relief here.  The format command needs to be run interactively.  It wants you to answer yes or no.  For once, Microsoft’s efforts to save us from ourselves have been useful.

xp_cmdshell 'diskpart'

And thankfully, by default, diskpart also runs interactively:

Here’s where it can get interesting.  Diskpart runs interactively by default, but can also run in scripted mode.  Let’s jump to a DOS box for a minute to see how that works:

I’ve just deleted the contents of disk 1.  It didn’t take much effort, did it?  Let’s see if we can script that.

First, create a text file somewhere that contains this:

select disk 1
clean all

Now, let’s run diskpart scripted:

Hmm.

I wonder if we can do the same thing with xp_cmdshell?

First, let’s create the command file:

exec xp_cmdshell 'echo select disk 1 > c:\diskpartcommands.txt'
exec xp_cmdshell 'echo clean all >> c:\diskpartcommands.txt'

Then, let’s run diskpart scripted:

xp_cmdshell 'diskpart -s c:\diskpartcommands.txt'

If you don’t think a misconfigured xp_cmdshell is a bad thing, try this on a production system.  See how long you stay employed.

Some notes on the demo above.  I ran these commands using an account that was a member of the Administrators group on the local machine.  That’s why I didn’t have to do anything else special in order for xp_cmdshell to work.

Let’s say I have an unprivileged user that I want to be able to run xp_cmdshell commands.  I need to give them access to the command:

grant execute on xp_cmdshell to [sql\gpotest]

Let’s try this again, this time running it under the context of a standard user account:

xp_cmdshell 'whoami.exe'

Msg 15153, Level 16, State 1, Procedure xp_cmdshell, Line 1
The xp_cmdshell proxy account information cannot be retrieved or is invalid. Verify that the ‘##xp_cmdshell_proxy_account##’ credential exists and contains valid information.

SQL is thoughtful enough to protect us from ourselves.  But we want to JUST MAKE IT WORK!!!!!!!!!!!!!!

So, let’s do what every app developer I’ve ever met would do.  Let’s configure the proxy account using a privileged account – maybe even the app developer’s own account (which is, of course, a member of the Administrators group).  This can be done via the GUI or TSQL, but for this demo, I’ll show the TSQL:

use master
go
EXEC sp_xp_cmdshell_proxy_account 'SQL\sillydeveloper','password'

Now when we run the whoami command, we get this result:

Msg 229, Level 14, State 5, Procedure xp_cmdshell, Line 1
The EXECUTE permission was denied on the object ‘xp_cmdshell’, database ‘mssqlsystemresource’, schema ‘sys’.

OK, fine.  Let’s grant this unprivileged user the rights to use xp_cmdshell:

USE master
GRANT exec ON xp_cmdshell TO [SQL\gpotest]

Let’s run the whoami command again:

Yay!  It works!  Great, huh?

Now, let’s try that diskpart stuff again:

exec xp_cmdshell 'echo select disk 1 > c:\diskpartcommands.txt'
exec xp_cmdshell 'echo clean all >> c:\diskpartcommands.txt'

You’ll recall those commands created the diskpart script file.  Let’s then run the script file:

xp_cmdshell 'diskpart -s c:\diskpartcommands.txt'

Oh dear.

Those of you in the know might be thinking we could do “select disk 0” in our diskpart script and run it to trash the system volume.  Fortunately, diskpart protects us from ourselves:

If there’s a saving grace, it’s that the system volume can’t be trashed using this simple method.  But hey, we can still wreak havoc.  Let’s say we have a database called SuckedIn, whose files live in F: drive:

sp_helpfile

You know where I’m going with this, right?  If we don’t know the disk number, we can still use diskpart to identify the volume number, by changing the script to “list volume” to identify where F: drive lives:

Then we just change the diskpart script to:

select volume 2
clean all

And the result is data destruction:

Thankfully, we still can’t trash the system volume, but if someone trashed your database volumes, you’d be just as screwed.  Don’t let this happen to you!

In case you passed out from boredom while reading my post, here’s the important bits that led to this disaster:

1. The SQL service account is set as Local System
2. The SQL proxy account is a privileged account, i.e. one that has Administrator rights over the local machine
3. xp_cmdshell is enabled under these conditions.

That is all it takes to allow an otherwise regular, unprivileged user to HOSE YOUR SQL BOX!  All of this could have been prevented by using best practice, which in case you fell asleep again, is very simple:

1. Configure your SQL instance to use an unprivileged account for its service account
2. For the love of all that’s holy, use an unprivileged account for the SQL proxy account
3. Don’t enable xp_cmdshell unless you absolutely, positively have to.

To change the SQL Server service account, use the SQL Server Configuration Manager tool.  You’ll need to restart the SQL Server service, so schedule an outage window to do this.

Once you’ve done this, use this script to configure the SQL Proxy account.  Make sure this is an unprivileged account:

use master
go
EXEC sp_xp_cmdshell_proxy_account 'TEST\svc-sql-proxy','password'

This change takes effect straight away.  No outage required.

Now when you run the whoami account as an administrator, you’ll get this result:

If you run it as an unprivileged user, you’ll get this result:

Now try running the diskpart script as the unprivileged user.  It won’t work.  YAY for system security!

One of the things I find myself doing every day is teaching apps developers how to do stuff with these principles in mind:

1. Least privilege
2. Smallest attack surface

Unfortunately, most apps developers in my workplace:

1. Are administrators on both their workstations (I'm OK with this) and their development servers (not so OK with this), which means they:
2. Install heaps of shit they don't need, particularly on the dev server
3. Configure that shit to run under their own administrative user account (!!!!)
4. Forget they've installed that shit
5. After a period of time, don't remember if that shit is necessary for their code to run
6. Install all the server-side components of their client/server app onto one single dev server.  This blurs the lines.  Is it the web part of the app that performs task X, or is it the SQL part?  Who knows.  The apps developers don't.

So when the time comes to provision their production environment, I get requests like this:

1. Install Excel on the SQL box and the web server box, and actually any other box that our app uses, because we can't remember which box actually needs it
2. Enable xp_cmdshell on the SQL box.  We're not really sure why, but we need it.
3. And while you're at it, we need you to create scheduled tasks on all the boxes (because we can't remember what box does what), and that task needs admin rights because otherwise it doesn't work.  But we don't really know why it doesn't work.

I wish I was making this shit up.  But this is what I've spent the last week extracting from a particular group of people.  Of course, when I say NO to this sort of crap, it becomes my problem to come up with an alternative that uses least privilege, whilst installing the smallest possible amount of stuff to make it work.

I'll write a separate post about the xp_cmdshell thing, but for now I want to deal with the Excel thing.  I don't believe that any SQL server should have anything installed on it aside from SQL.  Excel is a user app, not a server app.  And I don't believe for a second that SQL "needs" it to import Excel data.

So.  Getting onto the heart of this post.  How do we get that Excel data into a SQL server database?

It would be nice if something like this would just work.  It doesn't:

SELECT * INTO XLImport3 FROM OPENDATASOURCE('Microsoft.Jet.OLEDB.4.0',
'Data Source=c:\downloads\ImportTest-XLS.xls;Extended Properties=Excel 8.0')...[Sheet1$]

This code was snaffled from a MS article, BTW, http://support.microsoft.com/kb/321686.  The article also makes reference to linked servers, but for my purposes I just want to run ad hoc queries on transient spreadsheets.

We get this error message:

Msg 7308, Level 16, State 1, Line 1
OLE DB provider 'Microsoft.Jet.OLEDB.4.0' cannot be used for distributed queries because the provider is configured to run in single-threaded apartment mode.
 

Hmm.  WTF does that mean?  Believe it or not, it's not because you need to do something about apartment mode, whatever the hell that is.  It's actually SQL's obscure way of complaining about the data source driver.  For 2007 and 2010 versions of Excel files, you need to install a provider that knows how to deal with these files.  By default, SQL 2008 R2 doesn't have one:

Get a hold of the Microsoft Access Database Engine 2010 Redistributable (don't worry about the "Access" in the title, just download it).  Make sure you get the 64-bit version if you're running SQL 2008 R2.

Install it.  If you get this message, it's because you have a 32-bit installation of Office or one of its components.  Naughty!  Uninstall it.

Uninstalling the Office components might require a reboot.  My experience with the Access driver is that you'll need to reboot your server again (or possibly just restart the SQL services – I haven't tried this in my test environment yet).  So if you're doing this on a production box, be sure to schedule an outage.

You'll see a new provider in the list:

Let's try running that TSQL code again:

SELECT * INTO XLImport3 FROM OPENDATASOURCE('Microsoft.Jet.OLEDB.4.0',
'Data Source=c:\downloads\ImportTest-XLS.xls;Extended Properties=Excel 8.0')...[Sheet1$]

We get the same error message again because now we are using the wrong provider.  Let's change that:

SELECT * FROM OPENROWSET(
'Microsoft.ACE.OLEDB.12.0',
'EXCEL 12.0;Database=c:\downloads\ImportTest-XLS.xls;HDR=YES;IMEX=1',
'SELECT * FROM [sheet1$]');

Now we get another error message, though thankfully less obscure:

Msg 15281, Level 16, State 1, Line 1
SQL Server blocked access to STATEMENT 'OpenRowset/OpenDatasource' of component 'Ad Hoc Distributed Queries' because this component is turned off as part of the security configuration for this server. A system administrator can enable the use of 'Ad Hoc Distributed Queries' by using sp_configure. For more information about enabling 'Ad Hoc Distributed Queries', see "Surface Area Configuration" in SQL Server Books Online.
 

OK.  Let's enable that, shall we?

sp_configure 'show advanced options', 1;
RECONFIGURE;
GO
sp_configure 'Ad Hoc Distributed Queries', 1;
RECONFIGURE;
GO

Let's try again:

Msg 7399, Level 16, State 1, Line 1
The OLE DB provider "Microsoft.ACE.OLEDB.12.0" for linked server "(null)" reported an error. Access denied.
Msg 7350, Level 16, State 2, Line 1
Cannot get the column information from OLE DB provider "Microsoft.ACE.OLEDB.12.0" for linked server "(null)".

Oh for Christ's sake.  What now?

There are some things that need to be enabled on the provider itself:

USE [master]
GO
EXEC master.dbo.sp_MSset_oledb_prop N'Microsoft.ACE.OLEDB.12.0', N'AllowInProcess', 1
GO
EXEC master.dbo.sp_MSset_oledb_prop N'Microsoft.ACE.OLEDB.12.0', N'DynamicParameters', 1
GO
EXEC master.dbo.sp_MSset_oledb_prop N'Microsoft.ACE.OLEDB.12.0', N'NestedQueries', 1
GO
EXEC master.dbo.sp_MSset_oledb_prop N'Microsoft.ACE.OLEDB.12.0', N'NonTransactedUpdates', 1
GO

Most articles I've seen recommend just the first two settings, but in all my experiments I just couldn't get it to work.  I found an article that mentioned the last two settings and had success with that.  Your mileage may vary.

Let's try it again!

SELECT * FROM OPENROWSET(
'Microsoft.ACE.OLEDB.12.0',
'EXCEL 12.0;Database=c:\downloads\ImportTest-XLS.xls;HDR=YES;IMEX=1',
'SELECT * FROM [sheet1$]');

Holy snapping duckshit!  It WORKS!

You'll also be able to read .XLSX files:

SELECT * FROM OPENROWSET(
'Microsoft.ACE.OLEDB.12.0',
'EXCEL 12.0;Database=c:\downloads\ImportTest-XLSX.xlsx;HDR=YES;IMEX=1',
'SELECT * FROM [sheet1$]');

And CSV files, though I suspect you'd be able to do this without the driver:

 SELECT * FROM OPENROWSET(
'Microsoft.ACE.OLEDB.12.0',
'Text;Database=C:\Downloads\;HDR=Yes;',
'SELECT * FROM [ImportTest-CSV.csv]');

Note the syntax for a CSV is slightly different.  Also note that in all of them, the last parameter makes reference to the sheet name.  So make sure you've got that last part right.

So, in summary:

1. Remove any existing Office components and reboot.
2. Install the Microsoft Access Database Engine 2010 Redistributable and reboot
3. Run the following TSQL commands:

USE [master]
GO

sp_configure 'show advanced options', 1;
RECONFIGURE;
GO
sp_configure 'Ad Hoc Distributed Queries', 1;
RECONFIGURE;
GO

EXEC master.dbo.sp_MSset_oledb_prop N'Microsoft.ACE.OLEDB.12.0', N'AllowInProcess', 1
GO
EXEC master.dbo.sp_MSset_oledb_prop N'Microsoft.ACE.OLEDB.12.0', N'DynamicParameters', 1
GO
EXEC master.dbo.sp_MSset_oledb_prop N'Microsoft.ACE.OLEDB.12.0', N'NestedQueries', 1
GO
EXEC master.dbo.sp_MSset_oledb_prop N'Microsoft.ACE.OLEDB.12.0', N'NonTransactedUpdates', 1
GO

And you are good to go!!!!!!!!!!!!!!!! 

Now that you can actually read the damn files, you can import them into a table:

SELECT * INTO SomeTable
FROM OPENROWSET(
'Microsoft.ACE.OLEDB.12.0',
'EXCEL 12.0;Database=c:\downloads\ImportTest-XLS.xls;HDR=YES;IMEX=1',
'SELECT * FROM [sheet1$]');

Note that the only change in the query is the "select into" part.  The rest is the same, and this applies to the XLSX or CSV import.

The SELECT INTO will overwrite your table, so if the plan is to add rows to it, use this sort of query:

INSERT INTO SomeTable
SELECT * FROM
OPENROWSET(
'Microsoft.ACE.OLEDB.12.0',
'EXCEL 12.0;Database=c:\downloads\ImportTest-XLSX.xlsx;HDR=YES;IMEX=1',
'SELECT * FROM [sheet1$]');