What Is Cross-Site Scripting (XSS)? Understanding a Critical Web Security Vulnerability.

Imagine a website where users can post comments. Someone submits this as their comment: <script> alert ( " Hello " ); </script> If the application takes that input and places it directly into the HTML it serves to other users, the browser doesn't see a comment. It sees a script tag. The vulnerability isn't that JavaScript exists on the page. JavaScript belongs on web pages. The problem is that untrusted user input ended up in a context where the browser interpreted it as executable content rather than inert text. The Core Problem A browser rendering a webpage doesn't distinguish between HTML the developer wrote and HTML that arrived through a comment field. It parses what it's given. If user input gets embedded into the page without being handled carefully, the browser processes it the same way it processes everything else. User Input ↓ Web Application ↓ HTML / DOM ↓ Browser ↓ Input interpreted as executable content Untrusted data should remain data. XSS occurs when the application allows that data to cross into a context where the browser interprets it as code or executable markup. The boundary between "string containing angle brackets" and "HTML the browser will parse" is where the vulnerability lives. Three Forms of XSS XSS shows up in a few different ways depending on where the injection happens and how the input travels. Stored XSS is when untrusted input gets saved to a database and later served to other users. The comment example above is stored XSS. An attacker submits input once, and every user who views that page subsequently receives it. The application acts as an unwitting distribution mechanism. Reflected XSS involves input that isn't stored but gets reflected back in an immediate server response. Search pages are a common example: if a query is echoed into the page as "You searched for: [query]" and the query isn't handled carefully, an attacker can craft a URL whose query parameter contains a payload. When another user visits that URL, the server refl