SQL Server must limit the number of concurrent sessions to an organization-defined number per user for all accounts and/or account types.

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-213929	SRG-APP-000001-DB-000031	SQL6-D0-003600	SV-213929r1018580_rule	2025-02-12	3

Description
Database management includes the ability to control the number of users and user sessions utilizing SQL Server. Unlimited concurrent connections to SQL Server could allow a successful denial-of-service (DoS) attack by exhausting connection resources; and a system can also fail or be degraded by an overload of legitimate users. Limiting the number of concurrent sessions is helpful in reducing these risks. This requirement addresses concurrent session control for a single account. It does not address concurrent sessions by a single user via multiple system accounts. The capability to limit the number of concurrent sessions per user must be configured in or added to SQL Server (for example, by use of a logon trigger), when this is technically feasible. Note that it is not sufficient to limit sessions via a web server or application server alone, because legitimate users and adversaries can potentially connect to SQL Server by other means. The organization will need to define the maximum number of concurrent sessions by account type, by account, or a combination thereof. In deciding on the appropriate number, it is important to consider the work requirements of the various types of users. For example, 2 might be an acceptable limit for general users accessing the database via an application; but 10 might be too few for a database administrator using a database management GUI tool, where each query tab and navigation pane may count as a separate session. (Sessions may also be referred to as connections or logons, which for the purposes of this requirement are synonyms.)

Description

Database management includes the ability to control the number of users and user sessions utilizing SQL Server. Unlimited concurrent connections to SQL Server could allow a successful denial-of-service (DoS) attack by exhausting connection resources; and a system can also fail or be degraded by an overload of legitimate users. Limiting the number of concurrent sessions is helpful in reducing these risks. This requirement addresses concurrent session control for a single account. It does not address concurrent sessions by a single user via multiple system accounts. The capability to limit the number of concurrent sessions per user must be configured in or added to SQL Server (for example, by use of a logon trigger), when this is technically feasible. Note that it is not sufficient to limit sessions via a web server or application server alone, because legitimate users and adversaries can potentially connect to SQL Server by other means. The organization will need to define the maximum number of concurrent sessions by account type, by account, or a combination thereof. In deciding on the appropriate number, it is important to consider the work requirements of the various types of users. For example, 2 might be an acceptable limit for general users accessing the database via an application; but 10 might be too few for a database administrator using a database management GUI tool, where each query tab and navigation pane may count as a separate session. (Sessions may also be referred to as connections or logons, which for the purposes of this requirement are synonyms.)

ℹ️ Check
Review the system documentation to determine whether any concurrent session limits have been defined. If it does not, assume a limit of 10 for database administrators and 2 for all other users. If a mechanism other than a logon trigger is used, verify its correct operation by the appropriate means. If it does not work correctly, this is a finding. Due to excessive CPU consumption when utilizing a logon trigger, an alternative method of limiting concurrent sessions is setting the max connection limit within SQL Server to an appropriate value. This serves to block a distributed denial-of-service (DDOS) attack by limiting the attacker's connections while allowing a database administrator to still force a SQL connection. In SQL Server Management Studio's Object Explorer tree: Right-click on the Server Name >> Select Properties >> Select Connections Tab OR Run the query: EXEC sys.sp_configure N'user connections' If the max connection limit is set to 0 (unlimited) or does not match the documented value, this is a finding. Otherwise, determine if a logon trigger exists: In SQL Server Management Studio's Object Explorer tree: Expand [SQL Server Instance] >> Server Objects >> Triggers OR Run the query: SELECT name FROM master.sys.server_triggers; If no triggers are listed, this is a finding. If triggers are listed, identify the trigger(s) limiting the number of concurrent sessions per user. If none are found, this is a finding. If they are present but disabled, this is a finding. Examine the trigger source code for logical correctness and for compliance with the documented limit(s). If errors or variances exist, this is a finding. Verify that the system does execute the trigger(s) each time a user session is established. If it does not operate correctly for all types of user, this is a finding.

ℹ️ Check

Review the system documentation to determine whether any concurrent session limits have been defined. If it does not, assume a limit of 10 for database administrators and 2 for all other users. If a mechanism other than a logon trigger is used, verify its correct operation by the appropriate means. If it does not work correctly, this is a finding. Due to excessive CPU consumption when utilizing a logon trigger, an alternative method of limiting concurrent sessions is setting the max connection limit within SQL Server to an appropriate value. This serves to block a distributed denial-of-service (DDOS) attack by limiting the attacker's connections while allowing a database administrator to still force a SQL connection. In SQL Server Management Studio's Object Explorer tree: Right-click on the Server Name >> Select Properties >> Select Connections Tab OR Run the query: EXEC sys.sp_configure N'user connections' If the max connection limit is set to 0 (unlimited) or does not match the documented value, this is a finding. Otherwise, determine if a logon trigger exists: In SQL Server Management Studio's Object Explorer tree: Expand [SQL Server Instance] >> Server Objects >> Triggers OR Run the query: SELECT name FROM master.sys.server_triggers; If no triggers are listed, this is a finding. If triggers are listed, identify the trigger(s) limiting the number of concurrent sessions per user. If none are found, this is a finding. If they are present but disabled, this is a finding. Examine the trigger source code for logical correctness and for compliance with the documented limit(s). If errors or variances exist, this is a finding. Verify that the system does execute the trigger(s) each time a user session is established. If it does not operate correctly for all types of user, this is a finding.

✔️ Fix
If a trigger consumes too much CPU, an example alternative method for limiting the concurrent users is setting the max connection limit on SQL Server. In SQL Server Management Studio's Object Explorer tree, right-click on the Server Name >> Select Properties >> Select Connections Tab >> Set the Maximum Number of Concurrent Connections to a value other than 0 (0 = unlimited), and document it. OR Run the query: EXEC sys.sp_configure N'user connections','5000' /* this is an example max limit / GO RECONFIGURE WITH OVERRIDE GO Restart SQL Server for the setting to take effect. Reference: https://learn.microsoft.com/en-us/sql/database-engine/configure-windows/configure-the-user-connections-server-configuration-option? Otherwise, establish the limit(s) appropriate to the type(s) of user account accessing the SQL Server instance, and record them in the system documentation. Implement one or more logon triggers to enforce the limit(s), without exposing the dynamic management views to general users. Example script below: CREATE TRIGGER SQL_STIG_Connection_Limit ON ALL SERVER WITH EXECUTE AS 'renamed_sa' /Make sure to use the renamed SA account here/ FOR LOGON AS BEGIN If (Select COUNT(1) from sys.dm_exec_sessions WHERE is_user_process = 1 AND original_login_name = ORIGINAL_LOGIN() ) > (CASE ORIGINAL_LOGIN() WHEN 'domain/ima.dba' THEN 150 /this is a busy DBA’s domain account / WHEN 'application1_login' THEN 6 / this is a SQL login for an application / WHEN 'application2_login' THEN 20 / this is a SQL login for another application / … ELSE 1 / All unspecified users are restricted to a single login */ END) BEGIN PRINT 'The login [' + ORIGINAL_LOGIN() + '] has exceeded its concurrent session limit.' ROLLBACK; END END; Reference: https://msdn.microsoft.com/en-us/library/ms189799.aspx

✔️ Fix

If a trigger consumes too much CPU, an example alternative method for limiting the concurrent users is setting the max connection limit on SQL Server. In SQL Server Management Studio's Object Explorer tree, right-click on the Server Name >> Select Properties >> Select Connections Tab >> Set the Maximum Number of Concurrent Connections to a value other than 0 (0 = unlimited), and document it. OR Run the query: EXEC sys.sp_configure N'user connections','5000' /* this is an example max limit */ GO RECONFIGURE WITH OVERRIDE GO Restart SQL Server for the setting to take effect. Reference: https://learn.microsoft.com/en-us/sql/database-engine/configure-windows/configure-the-user-connections-server-configuration-option? Otherwise, establish the limit(s) appropriate to the type(s) of user account accessing the SQL Server instance, and record them in the system documentation. Implement one or more logon triggers to enforce the limit(s), without exposing the dynamic management views to general users. Example script below: CREATE TRIGGER SQL_STIG_Connection_Limit ON ALL SERVER WITH EXECUTE AS 'renamed_sa' /*Make sure to use the renamed SA account here*/ FOR LOGON AS BEGIN If (Select COUNT(1) from sys.dm_exec_sessions WHERE is_user_process = 1 AND original_login_name = ORIGINAL_LOGIN() ) > (CASE ORIGINAL_LOGIN() WHEN 'domain/ima.dba' THEN 150 /*this is a busy DBA’s domain account */ WHEN 'application1_login' THEN 6 /* this is a SQL login for an application */ WHEN 'application2_login' THEN 20 /* this is a SQL login for another application */ … ELSE 1 /* All unspecified users are restricted to a single login */ END) BEGIN PRINT 'The login [' + ORIGINAL_LOGIN() + '] has exceeded its concurrent session limit.' ROLLBACK; END END; Reference: https://msdn.microsoft.com/en-us/library/ms189799.aspx