What is the importance of filtering super-global?

Question

I wanted to know more details of what could happen when I stop filtering super global, mainly for data insertion. What loopholes could be opened? I started with php a little while ago and was making a small system just to learn and I showed the code to a colleague where I use a lot $_POST and $_GET he already has more experience and so he told me about the filters but I didn’t get it right.

Answer 1

Problem

When you build a system that works with a database, this is the biggest concern you should have about the data that is received by the user. For example, when you have a login form where the user passes email and password:

<form method="POST">
    Email: <input type="text" name="email" /><br />
    Senha: <input type="password" name="senha" /><br />
</form>

And in PHP you get the data this way:

<?php

$login = $_POST["login"];
$senha = $_POST["senha"];

$query = "SELECT * FROM usuarios WHERE login = '{$login}' AND senha = '{$senha}'";

?>

Malicious user wanting to hack into your system can simply type in the fields this ' OR 1 = '1 that the query executed would be:

SELECT * FROM usuarios WHERE login = '' OR 1 = '1' AND senha = '' OR 1 = '1'

Making the query catch the first user it finds and enter the system quietly.

Solution

To escape this type of problem that is common, believe (It is really easy to find systems with this type of failure), it is a good and simple practice to receive the form data this way:

<?php

$login = addslashes($_POST["login"]);
$senha = addslashes($_POST["senha"]);

$query = "SELECT * FROM usuarios WHERE login = '{$login}' AND senha = '{$senha}'";

?>

Where the user when trying to fill in the fields with this ' OR 1 = '1, would try to run this query:

SELECT * FROM usuarios WHERE login = '\' OR 1 = \'1' AND senha = '\' OR 1 = \'1'

That the database would interpret as 'Take all data of users whose login is equal to ' OR 1 = '1 and password equal to ' OR 1 = '1.

Running away from the bank attack.