Cross-Site Scripting (commonly referred as XSS) is one of the most prevalent and impactful vulnerability found in web applications. This may sound difficult, honestly, I was scared to even learn about it. But when I finally dug in, I realized its actual simple. Think of XSS as one of those spooky monsters you are afraid of as a kid, and then you turn on the light and realize its just your cat knocking over a lamp.

So, lets flip that light switch and break this concept down.

What is XSS?

It is a type of web vulnerability where an attacker sneaks their own code (usually JavaScript) into a website so that it runs in someone else's browser.

Basically:

  1. Website doesn't properly check user input
  2. Attacker slips in some sneaky code
  3. Another user loads the page and code runs in their browser

Types of XSS:

  1. Stored XSS
  • Attacker plants the malicious script in the website's database
  • Every time someone views that page, code runs
  • Example: Evil script hiding in a blog comment

2. Reflected XSS

  • The malicious script bounces off a server and comes back in the response
  • Typically happens in URLs or search queries
  • Example: Clicking on shady link and script runs instantly in browser

3. DOM based XSS

  • Here trickery happens entirely on the client side
  • No round trip to the server needed
  • Example: JavaScript on the page takes unvalidated input and directly injects it into page

Classic test payload:

To test for XSS vulnerability (legally, on system you own or have permission to test!) this script is usually used.

<script>alert(1)</script>

If a website is vulnerable, then this script will pop up a harmless alert box, thus confirming XSS vulnerability

Impact:

  • Steal cookies — hijack sessions, login as you
  • Key log your inputs — grab passwords, credit card numbers
  • Deface websites — changes what others see on page
  • Phishing — tricking users into giving up sensitive information

How to defend against it:

  • Input validation: Don't trust user input, sanitize it
  • Output encoding: Making sure special characters (<,>,", etc.) properly escaped
  • Use security headers
  • Use modern framework/libraries
  • Use tested libraries for sanitization

Final thoughts:

XSS may seem terrifying initially but once practiced few times it becomes easier to understand. Now that you have peeked under the bed, you cam see monster isn't really scary. It is just a little bug waiting for someone to either exploit it or squash it.

Disclaimer: These are for learning and legal testing only, don't go sprinkling test payloads on random websites unless you have explicit permission to test on them.