SQL Injection in Stored Procedure & Preventing from the same

Following is the small example of creating a stored procedure.

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CREATE PROC sp_login (@loginid nvarchar(25),@password nvarchar(25))
AS
DECLARE @SQLString VARCHAR(500)
DECLARE @loginid VARCHAR(64)
DECLARE @password VARCHAR(64)

/* Build the SQL string once.*/

SET @SQLString = 'SELECT * from cust_users WHERE login_id = '+ ''''+@loginid+'''' + 'AND password = '+ ''''+@password+''''

EXECUTE sp_executesql @SQLString

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Your ASP.NET Code would look like this:

oCmd.CommandText = “sp_login”;
oCmd.CommandType = CommandType.StoredProcedure;
oCmd.Parameters.Add( “@loginId”, strUserName);
oCmd.Paramerters.Add( “@password”, strPassword);
oCon.Open();
string result = (string)oCmd.ExecuteScalar();
oCon.Close();

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If the user input is as follows:
loginId = ‘ OR 1=1 —
password = junk

SQL injection will not work and ASP.NET will throw an exception

“Unclosed quotation mark after the character string ‘ OR 1=1 — and password=junk’.
Incorrect syntax near ‘ OR 1=1 — and password=junk’.”

In this case you can use

loginID = ” OR 1=1–

password = junk

Two single quotations are used to complete where clause with null condition and OR is used to make the condition true always.

If you use sp_executesql this will definitely leads to the SQL Injection.

See more on this http://msdn.microsoft.com/en-us/library/ms188001.aspx

Solution :

Instead one should use the same stored procedure which he has created, for passing parameters.

exec sp_login ‘param1’, ‘param2’

param1 – would be loginID

param2 – would be password

And you are stored procedure would look like this i.e. with out sp_executesql

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CREATE PROC sp_login

@loginid VARCHAR(64)
@password VARCHAR(64)

AS

BEGIN
SELECT * FROM cust_users WHERE loginid=@loginid AND password=@password
END

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This will avoid the possible SQL Injection

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