SQL Server Authorization and Permissions
Last Updated: October 26, 2023
Authorization in SQL Server involves controlling access to securable objects (like tables, views, stored procedures) based on the permissions granted to users and roles. This mechanism ensures that only authorized individuals can perform specific actions on your database.
Understanding Permissions
Permissions define what actions a user or role can perform on a particular object. These can be granted, denied, or revoked. Key concepts include:
- Principals: Logins and users that can be granted permissions.
- Securables: Objects within SQL Server that can have permissions applied to them.
- Permissions: Specific actions (e.g., SELECT, INSERT, EXECUTE) that can be allowed or denied.
Types of Permissions
Permissions are categorized into several types:
- Statement Permissions: Control the execution of specific SQL statements (e.g.,
CREATE TABLE,BACKUP DATABASE). - Object Permissions: Control access to specific database objects (e.g.,
SELECTon a table,EXECUTEon a stored procedure). - Application Role Permissions: Permissions managed through application roles.
Granting and Revoking Permissions
Permissions are managed using Transact-SQL (T-SQL) commands. The primary commands are:
GRANT: Used to give permissions.REVOKE: Used to remove permissions.DENY: Used to explicitly prohibit permissions.
Granting SELECT Permission on a Table
To grant the SELECT permission on a table named Customers to a user named AppUser, you would use the following T-SQL statement:
GRANT SELECT ON dbo.Customers TO AppUser;Revoking INSERT Permission on a Table
To revoke the INSERT permission on the same table from AppUser:
REVOKE INSERT ON dbo.Customers FROM AppUser;Denying DELETE Permission on a Table
To explicitly prevent AppUser from deleting rows:
DENY DELETE ON dbo.Customers TO AppUser;Important: DENY overrides GRANT. If a user is a member of a role that has been granted a permission, but the user has been explicitly denied that permission, the denial takes precedence.
Permissions Hierarchy
Permissions are hierarchical. For example, permissions granted at the server level might cascade down to databases, or permissions granted at the database level might apply to schemas and then to objects within the schema.
Managing Permissions with SQL Server Management Studio (SSMS)
SSMS provides a graphical interface for managing permissions, which can be more intuitive for many tasks:
- Connect to your SQL Server instance using SSMS.
- Navigate to the database, schema, or object you want to manage.
- Right-click on the object and select Properties.
- Go to the Permissions page.
- Here you can add users or roles and assign specific permissions.
Tip: Use fixed database roles (e.g., db_datareader, db_datawriter) and fixed server roles for common permission sets to simplify management.
Commonly Used Permissions
SELECT: Allows reading data from a table or view.INSERT: Allows adding new rows to a table.UPDATE: Allows modifying existing data in a table.DELETE: Allows removing rows from a table.EXECUTE: Allows running a stored procedure or function.ALTER: Allows changing the structure of an object (e.g., adding a column).CONTROL: Grants full control over an object, including the ability to grant and revoke permissions on it.
Best Practices
- Follow the principle of least privilege: Grant only the necessary permissions.
- Use roles to group users with similar access needs.
- Regularly audit permissions to ensure they are still appropriate.
- Avoid granting
sysadminordb_ownerroles unless absolutely necessary.
Caution: Incorrectly configured permissions can lead to security vulnerabilities or prevent legitimate users from performing their tasks.