Prevent dynamic SQL search from SQL injection - sql

Question

How can I make the following code SQL injection safe? I know that the problem is the following line:
SET #sqlCommand = #sqlCommand + 'Event.Name LIKE ' + '''%' + #name + '%'''
But I don't know how to make it SQL injection safe. I heard something about REPLACE but this doesn't solve the problem as a whole.
CREATE PROCEDURE searchEvents #name VARCHAR(50), #location VARCHAR(20), #postcode CHAR(4), #address VARCHAR(40), #startDate DATETIME, #endDate DATETIME
AS
DECLARE
#sqlCommand NVARCHAR(MAX) = 'SELECT Event.Name, Description, Location.Name AS Location, Postcode, Address, StartDate, EndDate, Website FROM Event JOIN Location ON Event.LocationID = Location.LocationID',
#parameters NVARCHAR(MAX),
#whereIncluded BIT = 0
BEGIN
IF #name IS NOT NULL
BEGIN
IF #whereIncluded = 0
BEGIN
SET #sqlCommand = #sqlCommand + ' WHERE '
SET #whereIncluded = 1
END
ELSE
SET #sqlCommand = #sqlCommand + ' AND '
SET #sqlCommand = #sqlCommand + 'Event.Name LIKE ' + '''%' + #name + '%'''
END
-- It's the same if clause for all parameters like above
SET #parameters = '#p_name VARCHAR(50), #p_location VARCHAR(20), #p_postcode CHAR(4), #p_address VARCHAR(40), #p_startDate DATETIME, #p_endDate DATETIME'
EXEC sp_executesql
#sqlCommand,
#parameters,
#p_name = #name,
#p_location = #location,
#p_postcode = #postcode,
#p_address = #address,
#p_startDate = #startDate,
#p_endDate = #endDate
END

Answer 1

Parametrise your SQL. Statements like SET #sqlCommand = #sqlCommand + 'Event.Name LIKE ' + '''%' + #name + '%''' are awful.
I'm not going to go too indepth here, there are 100's of example on how to make your SQL "safe(r)". HOwever, her's a few pointers...
Parametrisation:
Concatenating strings for variables is a sure way to leave yourself open to injection. Take the simple example below:
DECLARE #SQL nvarchar(MAX);
DECLARE #name varchar(1000);
SET #SQL = N'
SELECT *
FROM MyTable
WHERE [Name] = ''' + #Name + ''';';
EXEC (#SQL);
This is an awful example of Dynamic SQL. If you set the value of #name to ''; DROP TABLE MyTable;--' then SQL statement becomes:
SELECT *
FROM MyTable
WHERE [Name] = ''; DROP TABLE MyTable;-- ';
Oh good! Your table, MyTable has been dropped. The correct way would be:
DECLARE #SQL nvarchar(MAX);
DECLARE #name varchar(1000);
SET #SQL = N'
SELECT *
FROM MyTable
WHERE [Name] = #dName;';
EXEC sp_executesql #SQL, N'#dname varchar(1000)', #dName = #Name;
Dynamic Objects:
This is another common mistake people make. They have a query along the lines of:
DECLARE #SQL nvarchar(MAX);
DECLARE #Table varchar(1000);
SET #SQL = N'SELECT * FROM ' + #Table;
EXEC (#SQL);
This suffers exactly the same problem as above. You can't pass a variable as an object name, so you need to do this a little differently. This is by preferred method:
DECLARE #SQL nvarchar(MAX);
DECLARE #Table sysname; --notice the type here as well.
SELECT #SQL = N'SELECT *' + NCHAR(10) +
N'FROM ' + (SELECT QUOTENAME(t.[name]) --QUOTENAME is very important
FROM sys.tables t
WHERE t.[name] = #Table) + N';';
PRINT #SQL;
EXEC sp_executesql #SQL;
Why am I querying sys.tables? Well, this means if you do pass a nonsense table name in, the value of #SQL will be NULL; meaning that the dynamic SQL is completely harmless.
Like I said, this is just the basics; SO isn't the right place for a full answer here. There are 100's of articles on this subject, and you'll learn far more via your own research.

Answer 2

You can just...
SELECT
...
WHERE
#name IS NULL
OR Event.Name LIKE '%' + #name + '%'
You don't even need dynamic SQL in this case, so you can execute the above SQL directly, instead of through sp_executesql.
Be careful about the performance though - a LIKE operand starting with % may cause a full table (or clustered index) scan.