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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.

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.
Example
The following line of code illustrates this vulnerability
statement = “SELECT * FROM users WHERE name = ‘” + userName + “‘;”
This SQL code is designed to pull up the records of a specified username from its table of users. 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 ‘t’='t
renders this SQL statement by the parent language:
SELECT * FROM users WHERE name = ‘a’ OR ‘t’='t’;
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 ‘t’='t’ is always true.

Another Example
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.
For example:
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 users
will drop (delete) the “users” table from the database, since the SQL would be rendered as follows:
SELECT * FROM DATA WHERE id=1;DROP TABLE users;

Preventing SQL Injection

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

1. Using Parameterized Statements
In some programming languages such as Java and .NET parameterized statements can be used that work with parameters (sometimes called placeholders or bind variables) instead of embedding user input in the statement. In many cases, the SQL statement is fixed. The user input is then assigned (bound) to a parameter.
This is an example using Java and the JDBC API:

PreparedStatement prep = conn.prepareStatement(”SELECT * FROM USERS WHERE USERNAME=? AND PASSWORD=?”);
prep.setString(1, username);
prep.setString(2, password);

The same goes for C#:
using (SqlCommand myCommand = new SqlCommand(”select * from Users where UserName=@username and Password=@password”, myConnection))
{
myCommand.Parameters.AddWithValue(”@username”, user);
myCommand.Parameters.AddWithValue(”@password”, pass);

myConnection.Open();
SqlDataReader myReader = myCommand.ExecuteReader())
……………….
}

In ASP:
userName=replace(request.querystring(”username”))
userPassword=replace(request.querystring(”Password”))
userName=replace(userName,”‘”,”\’”)
‘userPassword it’s a numeric value.
userPassword=cdbl(userPassword)
myConn.execute “select * from users where name=’” & userName & _
“‘ and userPassword=” & userPassword & ” limit 1″

In PHP, it’s usual to just escape the parameters before sending the SQL query:
$query = sprintf(”SELECT * FROM Users where UserName=’%s’ and Password=’%s’”,
mysql_real_escape_string($Username),
mysql_real_escape_string($Password));
mysql_query($query);

For PHP version 5 and MySQL version 4.1 and above, however, you can use a vendor specific extension like msqli for “true” prepared statement queries.
$db = new mysqli(”localhost”, “user”, “pass”, “database”);
$stmt = $db -> prepare(”SELECT priv FROM testUsers WHERE username=? AND password=?”);
$stmt -> bind_param(”ss”, $user, $pass);
$stmt -> execute();
The mysql_real_escape_string adds backslashes (\) to escaped characters like single quotes ‘ and double quotes “. Though, you should read this

The magic_quotes_gpc PHP configuration directive affects Get, Post and Cookie values. If turned on, value (It’s “PHP!”) will automagically become (It\’s \”PHP!\”).

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Filed under: ASP.NET, C# Code, SQL Server Help by Rishav