CONTROL SERVER vs. sysadmin/sa:

permissions, system procedures, DBCC, automatic schema creation and privilege escalation caveats

Since SQL Server 2005, the server wide permission CONTROL SERVER has been existing. In principle being an alternative to sysadmin-membership, it did not turn out to be much more than a shelf warmer. – Little known and even less used.

One of the main reasons for this was the absence of an option to grant this permission to a group of principals/Logins on server-level.

Since SQL Server 2012, it has been possible to define custom server-roles, so now this permission is used more and more.

However, it is no complete replacement for sysadmin. In the following, I want to demonstrate to my readers exactly why this is, where the differences are and where it can even be a risk.

I will take as a given basic knowledge about the behavior of sysadmin and dbo and therefore focus on the things that are different when using CONTROL SERVER.

First of all, a new Login, „DBA_TheDude“, is created – the password of course adhering strongly to my rules (for blogs). 😉

This is followed by a new server role “Role_DBA” to which the Login will be added as a member.

And since this role is meant for administrators, it receives the most extensive permission: CONTROL SERVER.

Next we log onto the system as DBA and have “our (effective) rights” displayed. 🙂

 entity_name permission_name
server CONNECT SQL
server SHUTDOWN
server CREATE ENDPOINT
server CREATE ANY DATABASE
server CREATE AVAILABILITY GROUP
server ALTER ANY LOGIN
server ALTER ANY CREDENTIAL
server ALTER ANY ENDPOINT
server ALTER ANY LINKED SERVER
server ALTER ANY CONNECTION
server ALTER ANY DATABASE
server ALTER RESOURCES
server ALTER SETTINGS
server ALTER TRACE
server ALTER ANY AVAILABILITY GROUP
server ADMINISTER BULK OPERATIONS
server AUTHENTICATE SERVER
server EXTERNAL ACCESS ASSEMBLY
server VIEW ANY DATABASE
server VIEW ANY DEFINITION
server VIEW SERVER STATE
server CREATE DDL EVENT NOTIFICATION
server CREATE TRACE EVENT NOTIFICATION
server ALTER ANY EVENT NOTIFICATION
server ALTER SERVER STATE
server UNSAFE ASSEMBLY
server ALTER ANY SERVER AUDIT
server CREATE SERVER ROLE
server ALTER ANY SERVER ROLE
server ALTER ANY EVENT SESSION
server CONTROL SERVER

Sounds good. Nothing seems to be missing at first glance. Or does it?

Let’s compare it to the list of all permissions which exist on server-scope:

Ok, we really do have all permissions which can be granted (at server scope). This is also documented: Permissions (Database Engine) and here: GRANT Server Permissions (Transact-SQL)

Those permissions and commands really do work as documented.

But how about system stored procedures?

Why is that now?

Quite simple. Most do probably remember from SQL 2000 times, that security wise critical system procedures include a check for membership in certain roles. And most of them still do the same today. One can call it an artefact of the then inflexible permission-management.

When we look into the code of sp_readerrorlog, this is what is being done there as well:

Okay. Of course we can work around this code by calling xp_readerrorlog directly.

This does not help in the GUI-presentation in Management Studio but we can use it as a manual workaround.

Among other system procedures that are called via SSMS and refuse execution are: sp_addumpdevice – for backup-devices sp_enum_oledb_providers – is used for viewing and configuring Linked Servers.

sp_addlinkedserver on the other hand can be executed without problems – just not via the GUI because of the just mentioned procedure.

Setting up Database-Mail via the GUI is also reserved for sysadmins. It should work via script though (untested).

Also the configuration of the distributor for replication by means of sp_adddistributor is not possible without the sysadmin bit. Other replication tasks are partially possible even via the GUI.

Under SQL Server 2005 – 2008R2, to add/remove (server) role members, solely the system procedure sp_addsrvrolemember / sp_dropsrvrolemember exists. This one also conducts a server-role-membership-check. – Under SQL Server 2012 this procedure still exists for backwards-compatibility reasons, but this specific check has been removed from the code. Thereby it now behaves like the direct call of ALTER SERVER ROLE {RoleName} ADD MEMBER {Loginname}.

Same applies to sp_addrolemember / sp_droprolemember for database roles.

So much for system stored procedures. Quite simple after all, if you know the background.

Therefore, wherever possible: use DDL-commands, as it is advised since SQL Server 2005.

Side-note: there are at least 7 variations for the check of sysadmin-membership. 🙂 🙁

A teaser:

The most frequent error messages which are triggered if applicable are:

message_id text
14126 You do not have the required permissions to complete the operation.
14260 You do not have sufficient permission to run this command. Contact your system administrator.
15003 Only members of the %s role can execute this stored procedure.
15247 User does not have permission to perform this action.
21089 Only members of the sysadmin fixed server role can perform this operation.
22904 Caller is not authorized to initiate the requested action. DBO privileges are required.

A complete list of procedures that check for sysadmin-membership is included at the end of this post.

Let’s come to another important area:

DBCC commands

That’s a good start, isn’t it..

The „rule“ for DBCC is even simpler: Almost every DBCC-command checks for sysadmin.

From DBCC CHECKDB via DBCC LOGINFO to DBCC TRACEON. Thus some quite valuable commands also for external support. – In fact even DBCC HELP is only allowed for sysadmins

The only exceptions known to me are: DBCC SHOW_STATISTICS

  • one needs to be at least ddl_admin or owner of the table

DBCC DETACHDB

  • this is a special case, since it’s executed via sp_detach_db
  • db_owner or dbo of the database is sufficient

DBCC FREEPROCCACHE and DBCC SQLPERF

  • Those require only the ALTER SERVER STATE permission.

DBCC DROPCLEANBUFFERS on the other hand requires sysadmin-role membership.

Those permissions are documented quite well, by the way.

Database permissions

With CONTROL SERVER, a principal has full access to all databases. Almost, but only almost like sysadmins, which are, as is well-known, mapped to “dbo”.

Logins with merely CONTROL SERVER permission are not(!) mapped to dbo.

The consequences of this can be seen in the following:

–>

SSMS:

What happened?

Do you remember the pre-SQL 2012 problem in which windows groups could not be assigned a default schema? And what happened if a developer forgot to specify a schema when creating an object?

Right. This is exactly the same issue: a schema with the Login-Name of the developer has been created. A true joy for later clean-up, to identify and drop the „right wrong” schemas. This Issue (one of the most voted Connect-Items) has been resolved with SQL Server 2012.

Unfortunately, the handling of the CONTROL SERVER permission has been overlooked to be fixed. I did create a connect item: „Login with CONTROL SERVER Permission Creating an Object without specifying Schema leads to creation of new Schema with Login-Name”. – Unfortunately too late. :-/

Probably an identical code-block is being used for that, since just like in the Windows-Group scenario, this implicit creation of the schema is uncatchable.

Conclusion: Always specify a schema when creating an object as a CONTROL SERVER granted principal – best: always specify a schema. (there are other reasons for that, too)

Privilege-Escalation-risk:

I want to finish off with a security-wise extremely important hint:

Logins with CONTROL SERVER permission can impersonate EVERY Login. ALL OF THEM. Thus also all sysadmins and sa itself! – And even if SQL authentication is not active!

And this is how this can be marvellously exploited:

Those who wants to protect themselves from this form of privilege escalation must DENY impersonating. – And why would you not want to do that since this eventually would put the whole concept of CONTROL SERVER in question (?!) – Because this is the final and crucial difference:

Sysadmin-members cannot be restricted anything via DENY. Yet principals with mere CONTROL SERVER permissions can.

This is how all plain DDL/DML commands work.

So, to say it clearly again, because this is vital:

A Login with CONTROL SERVER Permission only makes sense when at the same time he is explicitly denied impersonation of any sysadmin (or better: any Logins). Otherwise you may as well just make him sysadmin right away!

Unfortunately this only works via “DENY IMPERSONATE ON LOGIN::[Loginname]”. This is because IMPERSONATE is hierarchically a direct subordinate of CONTROL SERVER!

For a changing list of system administrators, one has a real problem, in case of not remembering this crucial Deny! (Next risk!)

– At the latest at this point one should address the Auditing feature of SQL Server.

My conclusion and lessons learned therefore are:

CONTROL SERVER itself is a consistent continuation of the efforts of the security team to make SQL Server even simpler and thereby more robust.

Because of the impersonate-permissions, a privilege escalation is really simple. In order to prevent this, one should be familiar with the permission hierarchy of SQL Server. – As always: “Knowledge leads to security.”

What really does hurt, is one of the best application cases: Permissions for support-personnel, internal or external. The missing possibility to grant permissions to DBCC directly throws a monkey wrench in our plans. If one does not want to create a procedure as wrapper for every important DBCC command, there is no way around sysadmin in those scenarios. (!)

In the end, a strict “separation of duties” is the iron goal.

Here you can read more about it – you should also check out the “SQL Server Separation of Duties Framework” on codeplex with many good ideas:

https://techcommunity.microsoft.com/t5/SQL-Server/Separation-of-Duties-for-DBA-s/ba-p/383915

Update 04-2014:

With SQL Server 2014 CONTROL Server becomes a bit safer. You can read more on the new possibilities here:

New Permissions in SQL Server 2014: IMPERSONATE ANY LOGIN, SELECT ALL USER SECURABLES, CONNECT ANY DATABASE and the old CONTROL SERVER

A displacement of „sa“ is not planned by the way, but with every release the scenarios where sa/sysadmin is imperatively needed become less.

Happy securing,


Andreas

Enclosed the list of all system procedures under SQL Server 2012 that check for sysadmin-role membership (171 – vs. 171 under SQL server 2008. SQL Server 2014 CTP 1 still the same):

(Hint: Altogether 197 system procedures check for server role membership like diskadmin or serveradmin and so on)

Module_Name
fn_yukonsecuritymodelrequired
sp_add_agent_parameter
sp_add_agent_profile
sp_adddatatype
sp_adddistributiondb
sp_adddistributor
sp_addqreader_agent
sp_addsubscriber
sp_addsubscriber_schedule
sp_addtabletocontents
sp_attachsubscription
sp_cdc_cleanup_change_table
sp_cdc_disable_db
sp_cdc_disable_table
sp_cdc_drop_job
sp_cdc_enable_db
sp_cdc_enable_table
sp_cdc_restoredb
sp_cdc_vupgrade
sp_certify_removable
sp_change_agent_parameter
sp_change_agent_profile
sp_change_subscription_properties
sp_change_users_login
sp_changedistpublisher
sp_changedistributiondb
sp_changedistributor_password
sp_changedistributor_property
sp_changemergesubscription
sp_changeqreader_agent
sp_changereplicationserverpasswords
sp_changesubscriptiondtsinfo
sp_checkinvalidivarticle
sp_copysubscription
sp_create_removable
sp_cycle_errorlog
sp_dbcmptlevel
sp_dbmmonitoraddmonitoring
sp_dbmmonitorchangealert
sp_dbmmonitordropalert
sp_dbmmonitordropmonitoring
sp_dbmmonitorhelpalert
sp_dbmmonitorhelpmonitoring
sp_dbmmonitorresults
sp_dbmmonitorupdate
sp_dbremove
sp_drop_agent_parameter
sp_drop_agent_profile
sp_dropdatatypemapping
sp_dropdistpublisher
sp_dropdistributiondb
sp_dropdistributor
sp_dropmergepullsubscription
sp_droppullsubscription
sp_dropsubscriber
sp_dsninfo
sp_enumdsn
sp_flush_commit_table_on_demand
sp_generate_agent_parameter
sp_get_distributor
sp_get_Oracle_publisher_metadata
sp_getagentparameterlist
sp_getdefaultdatatypemapping
sp_grant_publication_access
sp_help_agent_default
sp_help_agent_parameter
sp_help_agent_profile
sp_helpdistpublisher
sp_helpdistributor
sp_helpmergesubscription
sp_helpqreader_agent
sp_helpreplicationdboption
sp_identitycolumnforreplication
sp_IHValidateRowFilter
sp_IHXactSetJob
sp_link_publication
sp_monitor
sp_MSadd_distribution_agent
sp_MSadd_logreader_agent
sp_MSadd_merge_agent
sp_MSadd_snapshot_agent
sp_MSadd_subscriber_schedule
sp_MSadd_tracer_history
sp_MSadd_tracer_token
sp_MScdc_cleanup_job
sp_MScdc_db_ddl_event
sp_MScdc_ddl_event
sp_MSchange_distribution_agent_properties
sp_MSchange_logreader_agent_properties
sp_MSchange_merge_agent_properties
sp_MSchange_snapshot_agent_properties
sp_MSchangedynamicsnapshotjobatdistributor
sp_MSchangedynsnaplocationatdistributor
sp_MScheck_pull_access
sp_MScleanupmergepublisher_internal
sp_MSclear_dynamic_snapshot_location
sp_MScreate_dist_tables
sp_MSdbuserpriv
sp_MSdeletefoldercontents
sp_MSdrop_6x_replication_agent
sp_MSdrop_merge_agent
sp_MSdrop_snapshot_dirs
sp_MSdropmergedynamicsnapshotjob
sp_MSdynamicsnapshotjobexistsatdistributor
sp_MSenumallpublications
sp_MSfetchAdjustidentityrange
sp_MSfix_6x_tasks
sp_MSforce_drop_distribution_jobs
sp_MSget_agent_names
sp_MSget_jobstate
sp_MSget_oledbinfo
sp_MSget_publication_from_taskname
sp_MSgetdbversion
sp_MSgetmaxsnapshottimestamp
sp_MShelp_repl_agent
sp_MShelp_replication_status
sp_MShelp_snapshot_agent
sp_MShelpconflictpublications
sp_MShelpdynamicsnapshotjobatdistributor
sp_MShelplogreader_agent
sp_MShelpsnapshot_agent
sp_MShelptranconflictcounts
sp_MSinit_publication_access
sp_MSreinit_failed_subscriptions
sp_MSremoveoffloadparameter
sp_MSrepl_backup_complete
sp_MSrepl_backup_start
sp_MSrepl_createdatatypemappings
sp_MSrepl_dropdatatypemappings
sp_MSrepl_enumarticlecolumninfo
sp_MSrepl_enumpublications
sp_MSrepl_enumpublishertables
sp_MSrepl_enumsubscriptions
sp_MSrepl_enumtablecolumninfo
sp_MSrepl_getdistributorinfo
sp_MSrepl_startup_internal
sp_MSreplagentjobexists
sp_MSreplcheck_permission
sp_MSreplcheck_pull
sp_MSreplcheck_subscribe
sp_MSreplcheck_subscribe_withddladmin
sp_MSreplcopyscriptfile
sp_MSreplremoveuncdir
sp_MSsetalertinfo
sp_MSSetServerProperties
sp_MSsetupnosyncsubwithlsnatdist
sp_MSsetupnosyncsubwithlsnatdist_cleanup
sp_MSsetupnosyncsubwithlsnatdist_helper
sp_MSstartdistribution_agent
sp_MSstartmerge_agent
sp_MSstartsnapshot_agent
sp_MSstopdistribution_agent
sp_MSstopmerge_agent
sp_MSstopsnapshot_agent
sp_MSupdate_agenttype_default
sp_oledbinfo
sp_procoption
sp_removedbreplication
sp_removesrvreplication
sp_replication_agent_checkup
sp_replicationdboption
sp_resetstatus
sp_restoredbreplication
sp_SetAutoSAPasswordAndDisable
sp_setdefaultdatatypemapping
sp_updatestats
sp_validatelogins
sp_vupgrade_mergeobjects
sp_vupgrade_replication
sp_vupgrade_replsecurity_metadata
xp_repl_convert_encrypt_sysadmin_wrapper

Tags:

Categories: SchemasSecurity

2 replies
  1. Zeshan Mujtaba
    Zeshan Mujtaba says:

    Good information but CONTROL SERVER is still very dangerous to be used. It can easily be escalated to an sa account or sysadmin, even if the IMPERSONATION on the account is disabled/denied.
    just like that the securityadmin server role should not be used either because of the CONTROL SERVER as anybody with securityadmin server role can easily escalate itself or any other account to sa or sysadmin.
    If needed I can provide a demonstration.
    Thank you.

    Reply
    • Andreas Wolter
      Andreas Wolter says:

      Hello Zeshan,
      yes, you are right. There are several ways to escape the CONTROL SERVER “boundaries” and elevate to sysadmin still left. The phrase I used was “more secure than before”. Maybe that is not sufficient. On the other hand I did not intend to show all PE techniques in public.
      If you think you know a way that is unknown to others/me, feel free to share with me privately and I will have a look.
      Improving security in SQL Server is now my current Job at Microsoft.
      bst regards
      Andreas

      Reply

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