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As a reminder, the following are two security concerns that are important in the case of dynamically assembled queries: Permissions on underlying tables SQL injection
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Permissions on Underlying Tables
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The fact that a caller has permission to execute the stored procedure that assembles the dynamic query does not mean that the caller has permission to access the underlying tables. You have to assign these permissions to the caller separately. Unfortunately, this requirement exposes your database someone might try to exploit the fact that you are allowing more than the execution of predefined stored procedures.
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SQL Injection
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Dynamically assembled queries present an additional security risk. A malicious user could use a text box to type something like:
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Acme' DELETE INVENTORY --
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A stored procedure (or application) can assemble this into a query, such as
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Select * from vInventory Where Make = 'Acme' DELETE INVENTORY --'
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The quote completes the parameter value and, therefore, the Select statement, and then the rest of the query is commented out with two dashes. Data from an entire table could be lost this way.
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SQL Server 2000 Stored Procedure & XML Programming
Naturally, a meticulous developer, such as you, would have permissions set to prevent this kind of abuse. Unfortunately, damage can be done even using a simple Select statement:
Acme' SELECT * FROM CUSTOMERS --
In this way, your competitor might get a list of your customers:
Select * from vInventory Where Make = 'Acme' SELECT * FROM CUSTOMERS --'
A hack like this is possible not just on string parameters; it might be even easier to perform on numeric parameters. A user can enter the following:
122121 SELECT * FROM CUSTOMERS
The result might be a query such as this:
Select * from vInventory Where InventoryId = 122121 SELECT * FROM CUSTOMERS
Fortunately, it s not too difficult to prevent this. No, you do not have to parse strings for SQL keywords. It s much simpler. The application must validate the content of text boxes. If a number or date is expected, the application must make sure that values really are of numeric or date data types. If text (such as a T-SQL keyword) is added, the application should prompt the user to supply a value of the appropriate data type. Unfortunately, if a text box is used to specify a string, there is little that you can validate. The key is to prevent the user from adding a single quote (') to the query. There are several ways to do this. The quote is not a legal character in some types of fields (such as keys, e-mails, postal codes, and so forth) and the application should not accept it in such fields. In other types of fields (such as company names, personal names, descriptions, and so on), use of quotes may be valid. In that case, in the procedure that assembles the string, you should replace a single quote char(39) with two single quotes char(39) + char(39) and SQL Server will find a match for the string:
set @chvMake = Replace(@chvMake, char(39), char(39) + char(39))
The dynamic query will become a query that works as expected:
Select * from vInventory Where Make = 'Dejan''s Computers Inc.'
9: Advanced Stored Procedure Programming
In the case in which someone tries to inject a SQL statement, SQL Server will just treat it as a part of the parameter string:
Select * from vInventory Where Make = 'Dejan'' SELECT * FROM CUSTOMERS --'
Another possibility is to replace a single quote char(39) with a character that looks similar onscreen but that is stored under a different code, such as (`) char(96). Naturally, you have to make this substitution for all text boxes (on both data entry and search pages). In some organizations, this substitution might be inappropriate, since existing databases may already contain quotes.
NOTE
You are at risk not only when you are passing strings directly from a GUI into the stored procedure that is assembling a query, but also when an application is reading the field with injected SQL into a variable that will be used in a dynamic query. A user might attempt to weaken security with some administrative procedure there is no need for direct interaction with the code by the attacker. Therefore, you should convert all string input parameters and local variables that are participating in the dynamic assembly of the query.
You also might want to prevent users from injecting special characters (such as wild cards) into strings that will be used in Like searches. The following function will make a string parameter safe for use in dynamic queries:
CREATE FUNCTION dbo.fnSafeDynamicString -- make string parameters safe for use in dynamic strings (@chvInput varchar(8000), @bitLikeSafe bit = 0) -- set to 1 if string will be used in LIKE RETURNS varchar(8000) AS BEGIN declare @chvOutput varchar(8000) set @chvOutput = Replace(@chvInput, char(39), char(39) + char(39)) if @bitLikeSafe = 1 begin -- convert square bracket set @chvOutput = Replace(@chvOutput, '[', '[[]') -- convert wild cards set @chvOutput = Replace(@chvOutput, '%', '[%]') set @chvOutput = Replace(@chvOutput, '_', '[_]') end RETURN (@chvOutput) END
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