Protection Against SQL Injection Attacks

In previous article ‘Passing parameters to query/ Stored Procedure’ of ADO.NET series, I recommended to use the SqlParameter to build the query based on the user input. The reason for the same was the SQL Injection.

What is SQL Injection?

SQL injection is a technique that exploits a security vulnerability (security system weakness) occurring in the database layer of an application. The weakness comes 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 weaknesses that can occur whenever one programming or scripting language is embedded inside another.

Incorrectly filtered escape characters:

This form of SQL injection occurs when user input is not filtered for escape characters and is then passed into a SQL statement. This results in the potential manipulation of the statements performed on the database by the end user of the application.

e.g.

The following line of code illustrates this vulnerability:

statement := "SELECT * FROM tEmployees WHERE name = '" + userName + "';"

This SQL code is designed to pull up the records of a specified username from its table of tEmployees. However, if the "userName" variable is crafted in a specific way by a malicious user, the SQL statement may do more than the code author intended. For example, setting the "userName" variable as

a' or 'n'='n’

renders this SQL statement by the parent language:

SELECT * FROM tEmployees WHERE name = 'a' OR 'n'='n';

If this code were to be used in an authentication procedure then this example could be used to force the selection of a valid username because the evaluation of 'n'='n' is always true.

While most SQL Server implementations allow multiple statements to be executed with one call.

Another example

The following value of "userName" in the statement below would cause the deletion of the " tEmployees" table as well as the selection of all data from the "data" table (in essence revealing the information of every user):

‘a’;DROP TABLE tEmployees; SELECT * FROM data WHERE name LIKE '%’

This input renders the final SQL statement as follows:

SELECT * FROM tEmployees WHERE name = 'a';DROP TABLE tEmployees; SELECT * FROM DATA WHERE name LIKE '%';

So putting subquery in the user input can be executed.

Incorrect type handling

This form of SQL injection occurs when a user supplied field is not strongly typed or is not checked for type constraints. This could take place when a numeric field is to be used in a SQL statement, but the programmer makes no checks to validate that the user supplied input is numeric.

e.g.

String statement = "SELECT * FROM data WHERE id = " + a_variable + ";"

It is clear from this statement that the author intended a_variable to be a number correlating to the "id" field. However, if it is in fact a string then the end user may manipulate the statement as they choose, thereby bypassing the need for escape characters. For example, setting a_variable to

1;DROP TABLE tEmployees

will delete the " tEmployees " table from the database, since the SQL would be rendered as follows:

SELECT * FROM DATA WHERE id=1;DROP TABLE tEmployees;

 How to Prevent SQL Injection?

To protect against SQL injection, user input must not directly be embedded in SQL statements. Instead, user input must be escaped, filtered, or parameterized statements must be used.

Using Parameters in the SqlCommand in ADO.NET would prevent the SQL injection attacks.

using (SqlCommand myCommand = new SqlCommand("select * from tEmployees where UserName=@username and Password=@password", myConnection))

                {                   

                    myCommand.Parameters.AddWithValue("@username", user);

                    myCommand.Parameters.AddWithValue("@password", pass);

                    myConnection.Open();

                    SqlDataReader myReader = myCommand.ExecuteReader());

                }

Summary: SQL Injection is a serious problem which should not be ignored by the programmer. It is easy to prevent the SQL injection using the SqlParametrs for the user input because it would filter the input and would not allow the subquery to be executed in the Database.