The Invisible Window - Uncovering the XSS Vulnerability

In the digital world, the threat is often not in what you see, but in what you fail to perceive. A website may look safe and functional, but beneath its surface it may hide a gap that allows an attacker to sneak in and manipulate the user's experience.

Today you will dive into the universe of Cross-Site Scripting (XSS), one of the most common and dangerous web vulnerabilities.

What is JavaScript?

JavaScript is the language that brings life and interactivity to web pages in the browser. It is the whisper behind every button, the heartbeat that makes the page respond to your actions.

What is JavaScript for?

• Create dynamic and responsive interfaces
• Validate forms in real time, without pauses or reloads
• Transform the content you see into something alive and changing
• Allow silent communication with servers, without you noticing

What is Cross-Site Scripting (XSS)?

XSS is the shadow that sneaks in when a web application allows malicious code, usually JavaScript, to infiltrate and take control in another user's browser.
It is the attacker's voice inside the guest's house, manipulating what should be safe, stealing what should not be touched.

Vulnerable code: an invitation to the attacker

Imagine a site that welcomes you, repeating your words without questioning them:

1<?php
2echo "Welcome, " . $_GET['user'];
3?>

Here, trust becomes vulnerability. Any user can leave a message that, disguised as innocent text, is actually a vehicle for injecting malicious code.

The art of the XSS payload is a simple code snippet that can trigger a security breach.

<script>alert('XSS')</script>

If it gets through, it will display a popup message. A small warning that the vulnerability is there, waiting to be exploited.

Types of XSS: the attacker's masks

• Stored XSS: The threat that hides and is stored, ready to attack anyone who discovers it.

• Reflected XSS: The threat that is instantly reflected, a dark mirror that returns the damage.

• DOM-based XSS: The threat that lives on the client side, manipulating the very structure of the content you see.

Understand to master

Think of an online store where customers' words should be sacred. But if you don't protect them, an attacker can hide a poisonous whisper among them, stealing secrets and control.

Find the path where this shadow hides in the code. It's not enough to see the wound; you must understand how it opened and what keeps it alive.

"Learn to listen to what is not said and you will master the invisible."