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

From MS SQL Server 2016 Instance Security Technical Implementation Guide

Part of SRG-APP-000001-DB-000031

Associated with: CCI-000054

SV-93825r1_rule SQL Server must limit the number of concurrent sessions to an organization-defined number per user for all accounts and/or account types.

Vulnerability discussion

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 per user 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; and it does not deal with the total number of sessions across all 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.)

Check content

Review the system documentation to determine whether any 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. Otherwise, determine if a logon trigger exists: In SQL Server Management Studio's Object Explorer tree: Expand [SQL Server Instance] >> Security >> 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 one(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 text

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. CREATE TRIGGER SQL_STIG_Connection_Limit ON ALL SERVER WITH EXECUTE AS '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() ) > 1000 --Organizationally defined number BEGIN PRINT 'The login [' + ORIGINAL_LOGIN() + '] has exceeded the concurrent session limit.' ROLLBACK; END END; Reference: https://msdn.microsoft.com/en-us/library/ms189799.aspx

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