Welcome to our blog post on SQL Server concepts! Today, we will be discussing the concept of Loginless Database Users and how they can be leveraged to maintain data security and preserve interface when using dynamic SQL within stored procedures.
Background
Dynamic SQL is a powerful feature in SQL Server, but it comes with its own set of challenges, including SQL injection, data security, plan cache re-use, database performance, and database design. While there are cases where dynamic SQL is justified, it is important to consider these factors before using it.
In this article, we will focus on maintaining a higher level of database security by using Loginless Database Users along with stored procedures that contain dynamic SQL.
Ownership Chaining
Ownership Chaining is a feature in SQL Server that allows users who have permission to an object to access other objects referenced within that object, as long as they are owned by the same database principal. This means that if a user has permission to execute a stored procedure, they can access the tables referenced within that stored procedure without needing explicit permissions on those tables.
However, when dynamic SQL is used within a stored procedure, the ownership chain is broken, and permissions need to be checked again. This means that the user executing the stored procedure would require additional permissions to access the tables referenced within the dynamic SQL.
The Solution: Loginless Database Users
To avoid granting direct access to tables referenced within dynamic SQL, we can use Loginless Database Users along with the WITH EXECUTE AS clause in stored procedures. Here are the steps:
- Create a low privilege login and database user that you want to allow to execute the stored procedure containing dynamic SQL, but whom you do not want to grant select permission to any tables.
- Grant permission to the new user to execute the stored procedure.
- Create a Loginless Database User and add it to a Database Role.
- Grant the role permissions to select from the tables referenced by the dynamic SQL.
- Alter the stored procedure to add a WITH EXECUTE AS clause, specifying the Loginless Database User.
By using this approach, the ownership chain is still broken within the stored procedure when the dynamic SQL runs, but the data access is performed under the security context of the Loginless Database User. This allows users to execute the stored procedure without granting them direct access to the tables referenced within the dynamic SQL.
Disadvantages and Alternatives
Using the WITH EXECUTE AS clause in stored procedures can have implications for auditing systems that are not designed to handle changes in security context during execution. It is important to consider the impact on auditing capabilities before implementing this technique.
An alternative to using WITH EXECUTE AS is signing stored procedures with a certificate, which also allows avoiding granting select permissions on referenced objects. This technique has its own set of properties and considerations, and it may be worth exploring as an alternative.
Conclusion
Stored procedures are an important component of securing data and implementing a successful data access strategy. By using Loginless Database Users and the WITH EXECUTE AS clause, developers can leverage dynamic SQL while still maintaining a consistent data access interface via stored procedures.
Thank you for reading this article on Loginless Database Users in SQL Server. We hope you found it informative and useful for your SQL Server projects.