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How to prevent SQL Injection in Stored Procedures

Shao Voon Wong
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4.78/5 (14 votes)

Apr 12, 2016

CPOL

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How to prevent SQL injection attacks when using dynamic SQL in stored procedures

Introduction

Some database programmers believe that by using stored procedures, their code are safe from SQL injection attacks. That is not entirely true if dynamic query is used inside the stored procedures and the dynamic query is constructed by concatenating the parameters. In circumstances, where the complicated query may use one, many, all or none of the parameters, it warrants the use of dynamic query. The easiest way to prevent SQL injection from happening, is to use parameters and sp_executesql to execute the dynamically generated search statement.

SQL Injection Attack

For illustration, this is the table and records, we will use for our examples. T-SQL is used in this tip.

CREATE TABLE tbl_Product
(
Name NVARCHAR(50),
Qty INT,
Price FLOAT
)

GO

INSERT INTO tbl_Product (Name, Qty, Price) VALUES (N'Shampoo', 200, 10.0);
INSERT INTO tbl_Product (Name, Qty, Price) VALUES (N'Hair Clay', 400, 20.0);
INSERT INTO tbl_Product (Name, Qty, Price) VALUES (N'Hair Tonic', 300, 30.0);

This is the stored procedure with dynamic query.

ALTER PROCEDURE sp_GetProduct(@Name NVARCHAR(50))
AS
BEGIN
    DECLARE @sqlcmd NVARCHAR(MAX);
    SET @sqlcmd = N'SELECT * FROM tbl_Product WHERE Name = ''' + @Name + '''';
	
    EXECUTE(@sqlcmd)
END

If @Name contains malicious string (see below) from the input of the C# program.

Shampoo'; DROP TABLE tbl_Product; --

The complete query string becomes

SELECT * FROM tbl_Product WHERE Name = 'Shampoo'; DROP TABLE tbl_Product; --'

The last quote sign is interpreted as comment. The tbl_Product is dropped. This can be prevented by denying the right to drop table to stored procedure caller.

It is obvious that a direct select (below) would suffice without using dynamic query. I could have used a complex query but the example is kept simple not to distract readers from the key point I am trying to drive home. 

ALTER PROCEDURE sp_GetProduct(@Name NVARCHAR(50))
AS
    SELECT * FROM tbl_Product WHERE Name = @Name; 

Solution 

The solution, as mentioned before, is to use parameters and sp_executesql. The second argument of sp_executesql should be set to the name and type of the parameters to expect in string form.

ALTER PROCEDURE sp_GetProduct(@Name NVARCHAR(50))
AS
BEGIN
    DECLARE @sqlcmd NVARCHAR(MAX);
    DECLARE @params NVARCHAR(MAX);
    SET @sqlcmd = N'SELECT * FROM tbl_Product WHERE Name = @Name';
    SET @params = N'@Name NVARCHAR(50)';
    EXECUTE sp_executesql @sqlcmd, @params, @Name;
END

Now the SQL injection fails after the sp_GetProduct is altered.

The C# calling code is not shown. Interested reader may download the VS2008 source code and T-SQL script.

Reference Books