If you worked -or still working- with SQL Server 2005 (or even earlier), you must have noticed that when you installed these SQL Server versions, the local Windows group “Built-In\Administrators” was automatically included in the SQL Server instance along with getting the role “SysAdmin” server role. This actually, when it comes to SQL Server security, is not a best practice.
For the above reason, from SQL Server 2008 and later this has stopped. One of the security changes in SQL Server 2008 (and later) was to stop automatically adding “Built-In\Administrators” as SQL Server SysAdmins during the SQL Server installation, thus leaving this decision to the person who performed the installation/setup of SQL Server and/or the Database Administrator (DBA), since it is a security risk for your SQL Server instance.
The above short introduction can easily lead us to the question: Should Windows “Built-In\Administrators” group be also SQL Server SysAdmins?
The above question has a definite answer based on SQL Server security best practices and that is No!
The above statement does not necessarily mean that a DBA cannot also have administrative access to the underlying machine onto which SQL Server is installed, but it basically suggests that the entire “Built-in\Administrators” group should never be included as “SysAdmins” in SQL Server. That is why SQL Server 2008 (and later) installation wizard does not automatically adds “Built-in\Administrators” as SQL Server SysAdmins anymore because in the end of the day, machine administrators and SQL Server administrators are two different roles that should not be mixed “by default”.
Another way to express the above concept in a single sentence is: A DBA can also be a machine administrator on a machine that has SQL Server installed on, but a machine administrator should not be a SQL Server SysAdmin.
*Important Note: At this point it is important to note that when installing SQL Server or handling security to never lock yourself out of the SQL Server instance. Always ensure that there is at least one active SysAdmin login mapped to a physical person (i.e. the DBA).
One of the available security checks in our SQL Server security tool “DBA Security Advisor“, is “Built-In\Administrators” access which checks and reports if there are any server roles assigned to the “Built-In\Administrators” group for the specified SQL Server instances.
Screenshot examples of checking the Built-In\Administrators access on a SQL Server 2017 instance:
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Reference: SQLNetHub (https://www.sqlnethub.com)