The SQL Server default account [sa] must be disabled.

SQL Server's [sa] account has special privileges required to administer the database. The [sa] account is a well-known SQL Server account and is likely to be targeted by attackers and thus more prone to providing unauthorized access to the database. This [sa] default account is administrative and could lead to catastrophic consequences, including the complete loss of control over SQL Server. If the [sa] default account is not disabled, an attacker might be able to gain access through the account. SQL Server by default disables the [sa] account at installation. Some applications that run on SQL Server require the [sa] account to be enabled for the application to function properly. These applications that require the [sa] account to be enabled are usually legacy systems.