Cross-Site Scripting (XSS)
A comprehensive guide to understanding, identifying, and preventing Cross-Site Scripting vulnerabilities in web applications.
What is Cross-Site Scripting?
Cross-Site Scripting (XSS) is a security vulnerability that allows attackers to inject malicious scripts into web pages viewed by other users. These scripts can steal sensitive information, hijack user sessions, deface websites, or redirect users to malicious sites.
XSS attacks occur when a web application includes untrusted data in a page without proper validation or escaping. The three main types are Reflected XSS, Stored XSS, and DOM-based XSS, each with unique characteristics and exploitation methods.
Types of XSS Vulnerabilities
Reflected XSS
HIGHMalicious script is reflected off a web server, typically via a URL parameter, form input, or search field. The attack is not persistent and requires the victim to trigger it.
EXAMPLE:
https://example.com/search?q=<script>alert(document.cookie)</script>Stored XSS
CRITICALMalicious script is permanently stored on the target server (database, message forum, comment field). The script is executed whenever a user accesses the affected page.
EXAMPLE:
Comment: <img src=x onerror="alert('XSS')">DOM-based XSS
HIGHThe vulnerability exists in client-side code rather than server-side. The attack payload is executed by modifying the DOM environment in the victim's browser.
EXAMPLE:
document.write(location.hash.substring(1))Mutation-based XSS (mXSS)
HIGHExploits the way browsers parse and render HTML. The payload mutates during sanitization or rendering, bypassing filters.
EXAMPLE:
<noscript><p title="</noscript><img src=x onerror=alert(1)>">Self-XSS
MEDIUMRequires the victim to input malicious content themselves, often through social engineering. Less severe as it requires user interaction.
EXAMPLE:
Tricking users to paste malicious code into browser consoleBlind XSS
CRITICALStored XSS where the attacker cannot see the result directly. Often targets admin panels or internal systems that process user input.
EXAMPLE:
Payload in support ticket that executes when admin views itPrevention Strategies
Input Validation
- ✓Validate all user input against expected formats
- ✓Use allowlists instead of denylists
- ✓Sanitize input on server-side
Output Encoding
- ✓HTML encode output data
- ✓Use context-appropriate encoding
- ✓Escape JavaScript strings properly
Content Security Policy
- ✓Implement strict CSP headers
- ✓Disable inline JavaScript execution
- ✓Use nonce or hash-based CSP
Security Headers
- ✓Set X-XSS-Protection header
- ✓Use X-Content-Type-Options
- ✓Enable HTTPOnly and Secure flags on cookies
Start Your Learning Journey
Master XSS security through structured, hands-on learning paths. Track your progress, complete challenges, and earn points.
XSS Fundamentals
4 hours • 5 lessons
Start your XSS security journey from the basics. Learn what Cross-Site Scripting is, how it works, and master fundamental attack vectors.
Offensive XSS Testing
12 hours • 0 lessons
Advanced exploitation and bypass techniques for penetration testing
Prerequisites: beginner
Defensive Strategies
10 hours • 0 lessons
Build secure applications and implement proper defenses against XSS
Prerequisites: beginner
Resources & Tools
OWASP XSS Guide
documentationComprehensive guide from OWASP on Cross-Site Scripting prevention and testing
PortSwigger XSS Cheat Sheet
cheatsheetExtensive collection of XSS vectors and filter bypass techniques
Content Security Policy (CSP)
documentationMDN documentation on implementing CSP to prevent XSS attacks
XSS Hunter
toolPlatform for finding blind XSS vulnerabilities
DOMPurify
toolFast, tolerant XSS sanitizer for HTML, MathML and SVG
HackerOne XSS Reports
guideReal-world XSS vulnerability reports and write-ups
Developer Tools
Professional security tools for testing and preventing XSS vulnerabilities. Built for developers and security researchers.
CSP Analyzer & Builder
AvailableParse and analyze Content Security Policy headers. Test XSS payloads against CSP rules. Build secure policies with framework-specific templates.
DOM Sink/Source Analyzer
AvailableScan JavaScript code for dangerous DOM sinks and untrusted sources. Visualize data flow from source to sink. Get remediation advice for vulnerable patterns.