Are you facing a dilemma when it comes to granting permissions to tables in your SQL Server database? Do you find it challenging to limit access to certain tables for specific users or roles? In this article, we will explore a solution to this problem using the DENY permission in SQL Server.

One common issue that many database administrators face is the use of all-inclusive roles like db_datareader, db_datawriter, and db_owner. These roles provide broad access to tables, making it difficult to restrict access to specific tables when needed. While the best approach is to re-architect the security permissions to grant the minimum necessary access for each user, this may not always be feasible due to time constraints.

Fortunately, SQL Server provides the DENY permission, which allows you to explicitly block access to tables for certain users or roles. Let’s take a look at an example to understand how this works.

-- Create a sample database and users
CREATE DATABASE [TestDB];
GO

-- Use the new database
USE [TestDB];
GO

-- Create users
CREATE USER [Reader] WITHOUT LOGIN;
CREATE USER [Writer] WITHOUT LOGIN;
CREATE USER [DBOwn] WITHOUT LOGIN;
GO

-- Add users to roles
EXEC sp_addrolemember @rolename = 'db_datareader', @membername = 'Reader';
EXEC sp_addrolemember @rolename = 'db_datawriter', @membername = 'Writer';
EXEC sp_addrolemember @rolename = 'db_owner', @membername = 'DBOwn';
GO

-- Create a sample table
CREATE TABLE dbo.SampleTable (SampleColumn INT);
GO

-- Grant implicit access using roles
EXECUTE AS USER = 'Reader';
SELECT SampleColumn FROM dbo.SampleTable;
REVERT;

EXECUTE AS USER = 'DBOwn';
SELECT SampleColumn FROM dbo.SampleTable;
REVERT;

EXECUTE AS USER = 'Writer';
INSERT INTO dbo.SampleTable (SampleColumn) VALUES (0);
REVERT;

EXECUTE AS USER = 'DBOwn';
INSERT INTO dbo.SampleTable (SampleColumn) VALUES (1);
REVERT;

-- Create a role and assign DENY permissions
CREATE ROLE [DenyAccess];
GO
DENY SELECT, INSERT, UPDATE, DELETE ON dbo.SampleTable TO DenyAccess;
GO
EXEC sp_addrolemember @rolename = 'DenyAccess', @membername = 'Reader';
EXEC sp_addrolemember @rolename = 'DenyAccess', @membername = 'Writer';
EXEC sp_addrolemember @rolename = 'DenyAccess', @membername = 'DBOwn';
GO

-- Test access after DENY
EXECUTE AS USER = 'Reader';
SELECT SampleColumn FROM dbo.SampleTable; -- Access denied
REVERT;

EXECUTE AS USER = 'Writer';
INSERT INTO dbo.SampleTable (SampleColumn) VALUES (2); -- Access denied
REVERT;

EXECUTE AS USER = 'DBOwn';
SELECT SampleColumn FROM dbo.SampleTable; -- Access denied
REVERT;

As you can see in the example above, by using the DENY permission, we can explicitly block access to the dbo.SampleTable for the users who are members of the DenyAccess role. Even though these users have implicit or explicit permissions through their roles, the DENY permission takes precedence and denies access to the table.

It’s important to note that while the DENY permission effectively restricts access for users with the db_datareader and db_datawriter roles, users with the db_owner role can still remove the DENY permission. However, in most cases, application users do not have the ability to remove DENY permissions, making this approach suitable for restricting access to tables in applications.

There is one exception to consider: the dbo user. The DENY permission cannot be assigned to the dbo user, and the dbo user cannot be made a member of a role. This means that you cannot block access to tables for the dbo user using the DENY permission. Additionally, members of the sysadmin server role are treated as dbo when accessing a database, so the DENY permission may not apply to them.

In conclusion, the DENY permission in SQL Server provides a powerful tool for restricting access to tables. By using the DENY permission, you can explicitly block access to tables for specific users or roles, even if they have implicit or explicit permissions through other roles. This approach allows you to maintain control over table access without the need for major security architecture changes.