Cross-Site Scripting (XSS): A Comprehensive Guide to Code Review
Table of Contents
Introduction
Cross-Site Scripting (XSS) remains one of the most common and dangerous web application vulnerabilities. According to the OWASP Top 10, XSS vulnerabilities continue to affect countless websites and applications, potentially exposing users to data theft, session hijacking, and malware distribution.
Did you know?
XSS has consistently appeared in the OWASP Top 10 since its inception, highlighting the persistent nature of this vulnerability class despite growing awareness.
This guide will help you understand how to identify XSS vulnerabilities during code review, explain how attackers exploit these weaknesses, and provide practical techniques to prevent them.
Which of the following best describes the main risk of XSS vulnerabilities?
What is XSS?
Cross-Site Scripting (XSS) is a vulnerability that allows attackers to inject malicious scripts into web pages viewed by other users. These scripts execute in the victim's browser, with access to cookies, session tokens, and other sensitive information retained by the browser for that site.
XSS Attack Flow
<script>alert("XSS")</script>store/reflect inputviews webpageCookies stolenActions performedXSS attacks occur when an application includes untrusted data in a webpage without proper validation or escaping.
Types of XSS Vulnerabilities
| Type | Description | Attack Vector |
|---|---|---|
| Reflected XSS | Script is reflected off a web server, such as in search results or error messages | URL parameters, form fields |
| Stored XSS | Malicious script is stored on the target server, such as in a database | Comments, user profiles, forums |
| DOM-based XSS | Vulnerability exists in client-side code rather than server-side | URL fragments, client-side storage |