Cross-Site Scripting (XSS) is a web security vulnerability that allows attackers to inject malicious client-side scripts into web pages viewed by other users. When a victim's browser executes the injected script, the attacker can steal session cookies, redirect users to malicious sites, modify page content, or perform actions on behalf of the victim.
There are three main types of XSS: Stored XSS (the malicious script is permanently stored on the target server), Reflected XSS (the script is reflected off the server in an error message or search result), and DOM-based XSS (the vulnerability exists in client-side code rather than server-side). Application security books on Amazon explain each type.
URL shortening services must guard against XSS in several areas: the link creation form (where users submit URLs), the custom alias input, the analytics dashboard, and any page that displays user-supplied content. Input validation, output encoding, and Content Security Policy headers form the primary defense layers.
Prevention best practices include encoding all user-supplied data before rendering it in HTML, using parameterized queries for database operations, implementing CSP headers, and using modern frameworks that automatically escape output. Secure coding books on Amazon cover defensive programming.