What is Cross-Site Scripting XSS - Complete Beginner Guide

Cross-Site Scripting, commonly abbreviated as XSS, represents one of the most prevalent and dangerous web application vulnerabilities that developers face today. At its core, XSS occurs when malicious scripts are injected into trusted websites and executed in users' browsers, effectively turning a legitimate website into a weapon against its own visitors. Imagine a scenario where you visit your favorite social media platform, but instead of just seeing your friend's posts, invisible malicious code runs in your browser, stealing your login credentials, hijacking your session, or even spreading the attack to your contacts. This is the reality of XSS attacks, and understanding them is crucial for anyone involved in web development or security.

The term "cross-site" in Cross-Site Scripting might seem confusing at first, as modern XSS attacks don't necessarily involve multiple sites. The name originates from early attacks where malicious scripts would cause browsers to send data across different sites, but today's XSS vulnerabilities are broader in scope. These attacks exploit the trust that users have in websites and the trust that browsers have in the code served by these websites. When a website fails to properly validate, sanitize, or escape user input before displaying it to other users, it creates an opportunity for attackers to inject malicious scripts that browsers will faithfully execute.