Avoid SQL injection

The most notorious one attack, well known but still available that
i have to explain and give a good solution for avoiding and protect
from it.

SQL injection is a technique that exploits a security vulnerability occurring in the database layer of an application. The vulnerability is present when user input is either incorrectly filtered for string literal escape characters embedded in SQL statements or user input is not strongly typed and thereby unexpectedly executed. It is in fact an instance of a more general class of vulnerabilities that can occur whenever one programming or scripting language is embedded inside another.

Know when we define it i will propose avoid/protect ways:

- Use Parameters with Stored Procedures:

Using stored procedures does not necessarily prevent SQL injection. The important thing to do is use parameters with stored procedures. If you do not use parameters, your stored procedures can be susceptible to SQL injection if they use unfiltered input as described in the "Overview" section of this document.

The following code shows how to use SqlParameterCollection when calling a stored procedure.

using System.Data;
using System.Data.SqlClient;

using (SqlConnection connection = new SqlConnection(connectionString))
{
DataSet userDataset = new DataSet();
SqlDataAdapter myCommand = new SqlDataAdapter(
           "LoginStoredProcedure", connection);
myCommand.SelectCommand.CommandType = CommandType.StoredProcedure;
myCommand.SelectCommand.Parameters.Add("@au_id", SqlDbType.VarChar, 11);
myCommand.SelectCommand.Parameters["@au_id"].Value = SSN.Text;

myCommand.Fill(userDataset);
}

- Use Parameters with Dynamic SQL

If you cannot use stored procedures, you should still use parameters when constructing dynamic SQL statements. The following code shows how to use SqlParametersCollection with dynamic SQL.

using System.Data;
using System.Data.SqlClient;

using (SqlConnection connection = new SqlConnection(connectionString))
{
DataSet userDataset = new DataSet();
SqlDataAdapter myDataAdapter = new SqlDataAdapter(
       "SELECT au_lname, au_fname FROM Authors WHERE au_id = @au_id",
       connection);              
myCommand.SelectCommand.Parameters.Add("@au_id", SqlDbType.VarChar, 11);
myCommand.SelectCommand.Parameters["@au_id"].Value = SSN.Text;
myDataAdapter.Fill(userDataset);
}
- Additional Considerations

Other things to consider when you develop countermeasures to prevent SQL injection include:

• Use escape routines to handle special input characters.
• Use a least-privileged database account.
• Avoid disclosing error information.

Use Escape Routines to Handle Special Input Characters

In situations where parameterized SQL cannot be used and you are forced to use dynamic SQL instead, you need to safeguard against input characters that have special meaning to SQL Server (such as the single quote character). If not handled, special characters such as the single quote character in the input can be utilized to cause SQL injection.

Note Special input characters pose a threat only with dynamic SQL and not when using parameterized SQL.

Escape routines add an escape character to characters that have special meaning to SQL Server, thereby making them harmless. This is illustrated in the following code fragment:

private string SafeSqlLiteral(string inputSQL)
{
return inputSQL.Replace("'", "''");

}