Cross-Site Scripting (XSS) remains one of the most prevalent web vulnerabilities in 2026, affecting applications across all industries. Understanding how XSS works and implementing proper defenses is crucial for any development team.
What is Cross-Site Scripting?
XSS attacks occur when an attacker injects malicious scripts into web pages viewed by other users. These scripts execute in the victim’s browser context, allowing attackers to steal session cookies, redirect users to malicious sites, or modify page content.
Types of XSS Attacks
Reflected XSS happens when user input is immediately returned by a web application without proper sanitization. The malicious script is part of the request and gets reflected back in the response.
Stored XSS occurs when malicious scripts are permanently stored on target servers, such as in databases or message forums. Every user who views the infected content executes the script.
DOM-based XSS exploits vulnerabilities in client-side JavaScript code that processes user input and updates the DOM without proper validation.
Modern Defense Strategies
Content Security Policy (CSP) headers provide a powerful defense layer by specifying which sources of content are trustworthy. Implementing strict CSP policies can prevent many XSS attacks from executing, even if malicious scripts make it into your pages.
Input validation should occur on both client and server sides. However, validation alone isn’t sufficient. All user input must be encoded appropriately for the context where it’s used, whether that’s HTML, JavaScript, CSS, or URL contexts.
Modern frameworks like React, Angular, and Vue provide built-in XSS protection through automatic output encoding. However, developers must understand when they bypass these protections using features like dangerouslySetInnerHTML or equivalent mechanisms.
Practical Implementation
Use HTTP-only cookies for session management to prevent JavaScript access to sensitive tokens. Implement strict input validation using allowlists rather than denylists. Deploy Web Application Firewalls (WAF) as an additional security layer, though never as your only defense.
Regular security testing, including automated scanning and manual penetration testing, helps identify XSS vulnerabilities before attackers do. Train your development team on secure coding practices and conduct code reviews with security in mind.
Conclusion
XSS prevention requires a defense-in-depth approach combining multiple strategies. By implementing proper output encoding, CSP headers, secure frameworks, and regular testing, you can significantly reduce your application’s XSS attack surface.