Programming Tutorials

What is SQL Injection

By: Emiley J. Printer Friendly Format    

SQL Injection is a method in which an attacker inserts malicious code into queries that run on your database. Have a look at this example:

<?php

$query = "SELECT login_id FROM users WHERE user='$user' AND pwd='$pw'";

mysql_query($query);

?>

Voilà! Anyone can log in as any user, using a query string like http://example.com/login.php?user=admin'%20OR%20(user='&pwd=')%20OR%20user=', which effectively calls the following statements:

<?php

$query = "SELECT login_id FROM users WHERE user='admin' OR (user = '' AND pwd='') OR user=''";

mysql_query($query);

?>

It’s even simpler with the URL http://example.com/login.php?user=admin'%23, which executes the query SELECT login_id FROM users WHERE user='admin'#' AND pwd=''. Note that the # marks the beginning of a comment in SQL.

Again, it’s a simple attack. Fortunately, it’s also easy to prevent. You can sanitize the input using the addslashes() function that adds a slash before every single quote ('), double quote ("), backslash (\), and NUL (\0). Other functions are available to sanitize input, such as strip_tags().