Spotting Unauthorized Configuration Settings in SQL Server

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Spotting Unauthorized Configuration Settings in SQL Server

Want to learn more about how to spot unauthorized configuration settings in your SQL server? Check out this post to learn more.

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An attacker of SQL Server likes to be able to change the SQL Server configuration settings. In an ideal world, you will have left everything open for the intruder, but generally, every DBA reduces the surface of attack as much as possible.

Why would the attacker want to change these settings? You might think there's limited risk in, for example, enabling 'cross-database ownership chaining,' or setting certain databases to 'Trustworthy,' for a small number of databases, but intruders can exploit some of these settings to enable features that gain access to objects in other databases, or even take control of the whole server. They can use them to ship your data off to a remote server.

If you've disabled these, and other sensitive settings, you might think that your SQL Server is well locked-down, but it is surprising how many attackers feel that they aren't really trying hard enough, if they don't manage to escalate their permissions sufficiently to gain ALTER SETTINGS server-level permission, at which point they can alter server and database configuration settings at will. The sysadmin and serveradmin fixed server roles already have this permission, and sometimes more staff than necessary, within the organization, are members of these roles. You really need to know when a change happens and who did it.

It may not be the sign of an attack when the configuration changes. More generally, it is disconcerting to discover how often configuration items can get changed. For example, I've experienced times when a developer has done a legitimate ad-hoc data extraction via xp_cmdshell and forgotten to lock down its use afterward.

In this article, I'll explain how to detect any changes in server or database configuration settings, using Extended Events and a custom metric in the SQL Monitor. I'll then refine the technique to report on unauthorized drift in the value of these settings by storing the officially agreed values in an extended property and comparing them to the current values.

Sensitive Server and Database Configuration Settings

Certain server configuration options only have a use in development. For a release candidate, these are generally closed off to reduce the attack surface. A certain number of these are particularly useful for an attacker, and so in a corporate setting, your security specialist will generally check these settings before signing off a release.

  • Ad Hoc Distributed Queries: If you were to set this on, users can then query data and execute statements on external data sources. It can be used remotely to access and exploit vulnerabilities on remote SQL Server instances.
  • CLR Enabled: Setting this may be essential if you are running CLR, but it introduces a risk, from both poorly-secured and malicious assemblies
  • Cross DB Ownership Chaining: This should never be turned on at the server level, but rather, it sould be enabled for specific databases only using the SETDB_CHAININGON. By enabling this, any member of the db_owner role in a database can gain access to objects owned by a login in any other database.
  • Database Mail XPs: Finding this enabled is ideal for the attacker because it allows them to exfiltrate data from the database server to a remote host, as an attachment to an email.
  • OLE Automation Procedures: This is used by attackers to execute functions within the server that hosts SQL Server within the security context of the SQL Server.
  • Remote Access: This can be used to launch a Denial-of-Service (DoS) attack on remote servers by off-loading query processing to a target.
  • Remote Admin Connections: This only needs to be enabled for SQL Server failover clusters
  • Scan For Startup Procs: If this is enabled, the SQL Server will automatically run all stored procedures that are configured to execute upon service startup. This allows an attacker to find a convenient way to maintain control of a server.
  • xp_cmdshell: This setting is one of the most popular for the attacker because it allows the authenticated SQL Server user to execute any operating-system command, Shell commands, and return results as rows within the SQL client. This would include being able to exfiltrate data out of SQL Server to a remote host.

There are two other sensitive, security options, which are not in the group of server configuration settings:

  • Trustworthy: This is a database-level external-access option. It allows CLR assemblies or extended procedures to access objects in other databases under certain circumstances. A user who is a member of the db_owner database role can exploit this to include themselves in the System Administrators server role, and so take control of the server. See 'Careful with Trustworthy Setting'
  • SA Account Status - this is a login setting. If you don't need to use this, because Windows Authentication is available, then it should be disabled. An attacker may try to enable it, in order to do a brute force attack.

How to Detect Server and Database Configuration Changes

SQL Server Audit will track changes to these configuration and security settings, but I need something more lightweight. Since one or two of the database settings can pose security issues, as well as many of the server settings, I decided on a single metric that covered both.

I describe a very simple approach to monitoring changes in the server and database configuration settings, but if you are just concerned with being alerted when a change is being made, it is sufficient. It relies on the fact that if you change any of these settings, an informational system message is sent, at severity level 10, which will tell you when an important configuration change was made along with the ID of the session.

Messages with levels 0 through 9 are classed as purely informational messages. They are sent to the user whose session made a change, but they aren't logged as errors. By contrast, changes to the database and server configuration settings trigger a Level 10 informational message, sometimes returned to applications as error-level 0, and they are logged as errors.

So, whenever a user executes sp_configure, to change a server configuration setting, a level 10 message is sent with the error number 15457. Similarly, a message with the error number 5084 is triggered every time a user issues ALTERDATABASE...SET..., to change a database configuration setting.

Since these messages appear in the error log, we can use SQL Server's alerts. They are also easy to pick up in an Extended Events event session, by capturing the sqlserver.error_reported event. It comes as a relief to find that we only need to filter on two error numbers because information messages, in general, are emitted from a working system at an alarming rate.

Using Extended Events to capture the two errors of interest is quicker than using the error log and gives you more information. It also allows you to add them to your security panel in SQL Monitor, as a custom metric. If you are quick, you'll also see them in SQL Monitor's display of the error log.

Setting up the Extended Events Session

To get this running, you need to create an Extended Events session. In this case, we can reuse the code from our last custom monitor, which I described when looking for SQL Injection-related errors in the article How to Detect SQL Injection Attacks using Extended Events and SQL Monitor. The same principle can be used here.

We'll collect all events that deal with server and database configuration changes, but no others.

IF EXISTS --if the session already exists, then delete it. We are assuming you've changed something
    SELECT * FROM sys.server_event_sessions
      WHERE server_event_sessions.name = 'ConfigurationItemsChanged'
    DROP EVENT SESSION ConfigurationItemsChanged ON SERVER;
  CREATE EVENT SESSION ConfigurationItemsChanged -- the name of the session 
    ADD EVENT sqlserver.error_reported --just the one event
         sqlserver.client_app_name, sqlserver.client_connection_id,
         sqlserver.database_name, sqlserver.nt_username, sqlserver.sql_text,
         sqlserver.username --all these are useful for tracking an error
    WHERE (([error_number]=(15457)) OR ([error_number]=(5084)))
    ADD TARGET package0.ring_buffer --we will rwrite it to a ring buffer targwet only
      (SET max_memory = (4096))

Listing 1

Once you have this in place, a ring buffer will store a list of configuration changes made since the session has started. We can then access the contents of the ring buffer for reporting, and the custom metric is a single SQL Query.

Detecting Config Changes in the SQL Monitor

For SQL Monitor, we only need to return an integer number that tells us how many configuration changes have happened in the past five minutes. Listing 2 shows the query to get that number from the ring buffer target of our ConfigurationItemsChanged event session.

SELECT Count(*) AS ErrorCount
    FROM sys.dm_xe_session_targets AS xet
      INNER JOIN sys.dm_xe_sessions AS xes
        ON xes.address = xet.event_session_address
      (SELECT Cast(xet.target_data AS XML)) AS target_data_xml(xml)
      CROSS APPLY target_data_xml.xml.nodes('//RingBufferTarget/event') AS xed(event_data)
    WHERE xes.name = 'ConfigurationItemsChanged'
      AND xet.target_name = 'ring_buffer'
      AND DateDiff
              Convert(DATETIMEOFFSET, xed.event_data.value('(@timestamp)[1]', 'datetime2')),
              DateName(TzOffset, SysDateTimeOffset())
           GetDate()) < 5;

Listing 2

Create this as a custom metric in SQL Monitor, as described in my previous article (referenced above), configure it to collect data on the master, and set it running to watch for changes! When errors are detected, the user can drill into the detail by looking in the error log or by using SQL to get more detailed information out of the ring buffer (I'll show this query shortly).

Making Some Configuration Changes

To check that all is well, we can twiddle some knobs and switches to change configuration settings (but only on a development server please!)

/* this code turns ON some of the configuration items with security issues */
   EXEC sp_configure 'show advanced options',1
  EXEC sp_configure 'CLR Enabled',1
  EXEC sp_configure 'Ad Hoc Distributed Queries',1 
  EXEC sp_configure 'Cross DB Ownership Chaining',1
  EXEC sp_configure 'Database Mail XPs',1
  EXEC sp_configure 'Ole Automation Procedures',1 
  EXEC sp_configure 'Remote ACCESS',1 
  EXEC sp_configure 'Remote Admin Connections',1 
  EXEC sp_configure 'Scan for Startup Procs',1 
  RECONFIGURE -- To update the currently configured value for advanced options.

Listing 3

And, this code turns them all off:

/* this code turns OFF some of the configuration items with security issues */
  EXEC sp_configure 'show advanced options',1
  EXEC sp_configure 'Ad Hoc Distributed Queries',0 
  EXEC sp_configure 'CLR ENABLED',0
  EXEC sp_configure 'Cross DB Ownership Chaining',0
  EXEC sp_configure 'Database Mail XPs',0
  EXEC sp_configure 'Ole Automation Procedures',0 
  EXEC sp_configure 'Remote ACCESS',0
  EXEC sp_configure 'Remote Admin Connections',0 
  EXEC sp_configure 'Scan for Startup Procs',0
  EXEC sp_configure 'show advanced options',0
  RECONFIGURE-- To update the currently configured value for advanced options.

Listing 4

Viewing the Metric and Alerts

Here's an analysis graph, plotting values for the configuration changed metric while I was testing!

If it spots a change and you've set it to fire an alert, you get this:

So, every time you make a config change, it will appear on the graph and fire an alert.

If you see something like this spike, which is just before the end at 9:45 AM, then you can drill down to see what is happening.

Retrieving the Details of the Configuration Changes

Now, we can see what happened and when using a more detailed query on our collected event data. We get more information than is in the error log because we can specify extra parameters, such as the identity of the client application.

/* now see what we have caught */
  DECLARE @Target_Data XML =
            SELECT TOP 1 Cast(xet.target_data AS XML) AS targetdata
              FROM sys.dm_xe_session_targets AS xet
                INNER JOIN sys.dm_xe_sessions AS xes
                  ON xes.address = xet.event_session_address
              WHERE xes.name = 'ConfigurationItemsChanged'
                AND xet.target_name = 'ring_buffer'
          SwitchOffset(CONVERT(datetimeoffset,xed.event_data.value('(@timestamp)[1]', 'datetime2')),
          DateName(TzOffset, SYSDATETIMEOFFSET()))) AS datetime_local,
  xed.event_data.value('(data[@name="error_number"]/value)[1]', 'int') AS [Error_Number],
  xed.event_data.value('(data[@name="severity"]/value)[1]', 'int') AS Severity,
  xed.event_data.value('(data[@name="message"]/value)[1]', 'varchar(255)') AS [Message],
  xed.event_data.value('(action[@name="username"]/value)[1]', 'varchar(255)') AS UserName,
  xed.event_data.value('(action[@name="nt_username"]/value)[1]', 'varchar(255)') AS NT_Username,
  xed.event_data.value('(action[@name="sql_text"]/value)[1]', 'nvarchar(max)') AS SQL_Text,
  xed.event_data.value('(action[@name="database_name"]/value)[1]', 'varchar(255)') AS [Database_Name],
  xed.event_data.value('(action[@name="client_connection_id"]/value)[1]', 'varchar(255)') AS client_conn,
  xed.event_data.value('(action[@name="client_app_name"]/value)[1]', 'varchar(255)') AS client_app_name
  FROM @Target_Data.nodes('//RingBufferTarget/event') AS xed (event_data)

Listing 5

In this case, it gives us the following:

Checking for Unauthorized 'Drift' in the Configuration Settings

We've done our monitoring the simple way so far, and it satisfies the basic requirement for alerting us about a change and gives us a baseline for how many changes are going on to the server and database configuration settings. The problem is that we get a false alert when the configuration is changed to what it should be, or if it is changed from 1 to 1 or 0 to 0. Also, if we somehow miss a change, we don't have a lasting account that the configuration is not what the team agreed it should be.

What we really want to know is: "have the configuration settings deviated in an unauthorized or uncontrolled way from what went through the deployment pipeline?" Beware that some database settings change for entirely benign system reasons, so it is just useful to be alerted when they happen, so you can check.

All you need to do is store a JSON document. The server configuration settings should be there and compare them to their current values. You can, of course, add a table with the default/agreed settings, but I prefer something less intrusive. I store them in a JSON table in an extended property. It won't store a lot of data, but there is plenty of space in an extended property for this sort of task. Of course, if you are running an old version of SQL Server, then you'll have to use a table to store this information or use XML instead of JSON.

Drift in Server Configuration Using a ServerConfig Extended Property

For the server configuration settings, we'll add an extended property called ServerConfig, which registers what the server properties should be. This code takes the current settings from the configuration system table ( sys.configurations).

USE master
  IF NOT EXISTS --if the record of the settings doesn't exist ....
   (SELECT * FROM ::fn_listextendedproperty
   BEGIN --if no record
   -- all we can do is insert initial configuration data
   -- so we create a JSON document that includes the date and time of insertion.
    (SELECT Convert(VARCHAR(7500),
      (SELECT Json_Query(g.r)AS r FROM
       (SELECT Convert(VARCHAR(7500),(SELECT f.date, f.configuration
             (GetDate(), -- put the current date and time in there
      --and the current configuration settings as a json array
             (SELECT cfg.configuration_id AS id, cfg.value AS v
                FROM sys.configurations cfg FOR JSON AUTO)--and the array
          )f(date,configuration) FOR JSON AUTO, WITHOUT_ARRAY_WRAPPER))
       )g(r) FOR JSON AUTO
   )--now write it into the extended property
   EXEC sp_addextendedproperty @name = N'ServerConfig',@value = @ThisConfig

Listing 6

Then, at any time, you can check to see the extent of deviation from the signed-off configuration.

USE master
  SELECT  id, Convert(BIGINT,old.value) AS CorrectValue, 
          Convert(BIGINT,live.value) AS CurrentValue, 
          live.description+ CASE WHEN is_advanced=1 THEN ' (advanced)' ELSE '' end  AS description
        FROM OpenJson((SELECT TOP 1 Convert(VARCHAR(MAX),value)
        FROM ::fn_listextendedproperty 
           ('ServerConfig', DEFAult, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT))) AS f --base array
        OUTER APPLY OpenJson(f.Value)r --value of the R variable
        OUTER APPLY OpenJson(r.Value)s --separate objects within r variable
        OUTER APPLY OpenJson(s.Value ) WITH (id INT '$.id',value INT '$.v') old
        INNER JOIN sys.configurations live
          ON old.id=live.configuration_id
        WHERE old.value<>live.value
          AND s.[Key]='configuration' -- there is also the date object.
          AND f.[Key] =0

Listing 7

To test it out, run Listing 6 to create ServerConfig, then Listing 3 to change some settings. Then, run Listing 7,  and finally, run Listing 4 to return all the settings to what they were.

Drift in a Database's Configuration Using a DBConfig Extended Property

Database configuration settings are stored at the database level, and again, we grab the registered values from sys.databases and pop them into the JSON table in an extended property, DBConfig, in the same way as server settings. We will need to do it separately for every database that we want to check on for changes to the configuration.

USE PhilFactor
  IF NOT EXISTS --if the record of the settings doesn't exist ....
    (SELECT * FROM ::fn_listextendedproperty 
    BEGIN --if no record
    -- all we can do is insert initial configuration data
    -- so we create a JSON document that includes the date and time of insertion.
    DECLARE @ThisConfig VARCHAR(7500) =
      (SELECT  json_query((SELECT * FROM sys.databases WHERE name LIKE Db_Name() FOR JSON AUTO)))
    --now write it into the extended property
    EXEC sp_addextendedproperty @name = N'DBConfig',@value = @ThisConfig 

Listing 8

Let's now make a dangerous change to the PhilFactor database settings.

  SET trustworthy on;  

Listing 9

Our SQL Monitor custom metric will immediately detect the problem, and we'll see a blip on the analysis graph for the metric and an alert. We can then investigate the changes by running this SQL Query.

SELECT [then].[key],[then].[value] AS Should_Be,[now].[value]AS [is] FROM 
  OpenJson((SELECT Json_Query(Convert(VARCHAR(max),
                 ( SELECT value FROM::fn_listextendedproperty(
     OUTER APPLY OpenJson(OriginalArray.Value)[then]
    OpenJson ((SELECT * FROM sys.databases 
                 WHERE name LIKE Db_Name() FOR JSON AUTO))CurrentArray
     OUTER APPLY OpenJson(CurrentArray.Value)[now]
  ON  [now].[Key]=[then].[Key]  
  WHERE [then].[value]<>[now].[value]

Listing 10

And see, we have a drift from what should be there.

Detecting drift in database settings across all databases

Listing 11 shows how to get a single report for all your databases. I would advise a complete check across all databases, because the 'Trustworthy' exploit can work, regardless of which database is set to be trustworthy.

Assuming you simply don't create the DBConfig extended property on any database you don't want to monitor, then the query won't report drift for those databases. Neat, eh? This will report before and after configuration changes for all databases on which you've installed DBConfig.

Beware that your database compatibility version needs to be 130 or above on whatever version of SQL Server you are running; the code will run, but it will then return a syntax error involving the OpenJson error for any databases of lower compatibility level.

DECLARE @DriftedSettings TABLE 
    ([KEY] sysname, ShouldBe VARCHAR(400), [Is] VARCHAR(400), [Database] sysname);
  INSERT INTO @DriftedSettings ([KEY], ShouldBe, [Is], [Database])
  EXEC sys.sp_MSforeachdb 'USE ? 
  SELECT [then].[key],[then].[value] AS Should_Be,[now].[value]AS [is] ,
  Db_Name() AS [Database]
  OpenJson((SELECT Json_Query(Convert(VARCHAR(max),
                 ( SELECT value FROM::fn_listextendedproperty(
     OUTER APPLY OpenJson(OriginalArray.Value)[then]
    OpenJson ((SELECT * FROM sys.databases 
                 WHERE name LIKE Db_Name() FOR JSON AUTO))CurrentArray
     OUTER APPLY OpenJson(CurrentArray.Value)[now]
  ON  [now].[Key]=[then].[Key]  
  WHERE [then].[value]<>[now].[value]
  SELECT drift.[KEY], drift.ShouldBe, drift.[Is], drift.[Database]
    FROM @DriftedSettings AS drift;

Listing 11

And, here's the report:


We now have a solution that gives us an alert when a server or database configuration changes on the server. When you notice activity, you can then see all the alterations, how it happened, when it happened, and who did it. You can then check to see whether it has resulted in the server or database configuration drifting from what it should be.

As with my previous article on spotting intrusion, I'm showing you enough code to inspire your own ideas. Please don't consider it a complete solution; intrusion detection is a moving target, and you will be more than one step ahead of the villains!

SQL, config, config settings, database, extended events session, security, sql server

Published at DZone with permission of Phil Factor . See the original article here.

Opinions expressed by DZone contributors are their own.

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