Introduction to SQL Injection

Understand the mechanics of one of the most critical web vulnerabilities and learn how poorly sanitized inputs can compromise entire databases.

The Anatomy of a Flaw

SQL Injection (SQLi) occurs when an application improperly handles user-supplied data before sending it to a backend database. Instead of treating the input as pure data, the database interprets it as executable code.

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Critical Concept

The core issue is a failure to separate Data from Context. When the interpreter cannot distinguish between the query structure and the user's input, arbitrary commands can be executed.

A Vulnerable Query

Consider a simple authentication mechanism. The backend code might construct a query like this:

PHP

$username = $_POST['user'];
$password = $_POST['pass'];

// VULNERABLE CODE - Direct concatenation
$query = "SELECT * FROM users WHERE username = '" . $username . ";

If an attacker submits admin' -- as the username, the constructed query becomes:

SELECT * FROM users WHERE username = 'admin' --'

The -- sequence comments out the rest of the query (e.g., password checking), bypassing authentication entirely.