When discussing and learning security related to SQL Server, it is always better to know the key concepts and terms related to it. Some of the concepts are not only related to SQL Server, they are related to almost all applications in terms of security. In simplest way, security is all about allowing someone or something to access a resource and perform actions on it. Here are some terms used when describing it:
Securables
Securable is mainly a resource which we can assign permissions. SQL Server has securables at multiple level of a hierarchical architecture. The hierarchy starts from the server level which includes securables like Endpoints, Logins, Server Roles and Databases. These securables are called as server-level securables as well as server scope securables.
Next level of the hierarchy is the database. Database-level or database scope securables includes items like Users, Database Roles, Certificates and Schemas.
SQL Server includes securables at Schema level too. They are called as schema scope securables that includes resources like Tables, Views, and Procedures.
Principals
The someone or something that perform actions on securables is called as a Principal. There are three types of principals related to SQL Server security: Window's Principals, SQL Server Principals, and Database Principals.
Window's principals and SQL Server principals are considered as server level principals. Windows level principals are generally domain or local server user accounts or groups that are used to connect with SQL Server instance. Authentication is done by either local server or domain controller, and SQL Server trusts the account without performing authentication. SQL Server level principals are logins created at SQL Server instance and authentication is done by SQL Server itself.
Database principals includes database users, database roles and application roles.
** Some principals are also securables. As an example, Login is a principal as well as a securable. It is a principal that because it can access the SQL Server instance and it is also a securable because there are actions that can be performed on it such as enabling and disabling that require permission.
Permissions
Permissions allow principals to perform actions on securables. There are two types of permissions related to SQL Server: Statement permission and Object Permission.
Statement permissions refer actions that can be performed by executing a statement. The principal creating a table using CREATE TABLE statement with CREATE TABLE permission is an example for it.
Object permissions refer actions that can be performed on securables. A principal having SELECT permission on a table is an example for this.
This image shows how these concepts are worked together. Note that the image has been taken from an article in TechNet Magazine. You can refer the article at: https://technet.microsoft.com/en-us/magazine/2009.05.sql.aspx
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