Cross-Site Scripting (XSS)
Last updated: January 12, 2026
XSS allows attackers to inject malicious scripts that execute in users' browsers, stealing sessions, credentials, or performing actions as the user.
Scan for This VulnerabilityWhat is Cross-Site Scripting (XSS)?
Cross-Site Scripting occurs when an application includes untrusted data in web pages without proper validation or encoding. The attacker's script runs in the victim's browser with full access to cookies, session data, and the ability to modify page content or make requests as the user.
Why It's Dangerous
This vulnerability can allow attackers to access sensitive data, compromise user accounts, or gain unauthorized control over your application. In AI-generated code, this issue is particularly common because security measures are often deprioritized in favor of rapid feature development.
Why AI Code Is Vulnerable
AI code generation tools focus on producing functional code quickly. They often generate patterns that work correctly but lack the defensive measures experienced security engineers would implement. This makes cross-site scripting (xss) particularly prevalent in vibe-coded applications.
Understanding the Technical Details
Cross-Site Scripting (XSS) is classified as a high-severity vulnerability because of its potential to cause significant damage to your application and users. Understanding the technical mechanics helps you recognize and prevent this issue in your own code.
This vulnerability typically occurs when security controls are either missing entirely, improperly configured, or incorrectly implemented. In many cases, the code appears to work correctly during development and testing, but the security flaw becomes exploitable once the application is deployed and accessible to malicious actors.
Attackers actively scan for this type of vulnerability using automated tools. Once discovered, exploitation can be rapid—often within hours of your application going live. The consequences range from data theft and account takeover to complete system compromise depending on the application's architecture.
For vibe-coded applications built with platforms like Lovable, Bolt.new, Replit, or v0.dev, this vulnerability appears in roughly 20-40% of deployments according to security research. The AI-generated patterns often follow insecure defaults that require manual security hardening.
How It Happens
- Rendering user input without encoding
- Using innerHTML with untrusted data
- Missing Content-Security-Policy header
- Improper URL parameter handling
Impact
Session hijacking through cookie theft
Credential theft via fake login forms
Malware distribution
Website defacement
Keylogging and data exfiltration
How to Detect
- Test inputs with script tags and event handlers
- Use XSS scanning tools
- Review code for innerHTML and dangerouslySetInnerHTML
- Run VAS to detect XSS vulnerabilities
How to Fix
Use framework encoding
React, Vue, and Angular encode output by default.
// React - safe by default
return <div>{userInput}</div>;
// BAD - bypasses encoding
return <div dangerouslySetInnerHTML={{__html: userInput}} />;Implement Content-Security-Policy
CSP prevents inline scripts from executing.
Content-Security-Policy: default-src 'self'; script-src 'self'Sanitize HTML if needed
Use a library like DOMPurify for user-generated HTML.
import DOMPurify from 'dompurify';
const clean = DOMPurify.sanitize(userHTML);Commonly Affected Platforms
Prevention Best Practices
The most effective approach to cross-site scripting (xss) is prevention. Implementing security measures during development is significantly easier and less costly than remediating vulnerabilities after deployment.
Security-First Development
When using AI code generation tools, always review the generated code for security implications. AI tools prioritize functionality over security, so treat all generated code as requiring security review. Establish a checklist of security requirements specific to your application type and verify each before deployment.
Continuous Security Testing
Integrate security scanning into your development workflow. Run scans after major code changes, before deployments, and on a regular schedule for production applications. Early detection of vulnerabilities reduces remediation costs and prevents potential breaches.
Defense in Depth
Never rely on a single security control. Implement multiple layers of protection so that if one control fails, others still protect your application. For example, combine authentication, authorization, input validation, and output encoding to create comprehensive protection against attacks.
Stay Informed
Security threats evolve constantly. Follow security researchers, subscribe to vulnerability databases, and monitor your dependencies for known issues. Understanding emerging threats helps you proactively protect your applications before attackers exploit new techniques.
Is Your App Vulnerable?
VAS automatically scans for cross-site scripting (xss) and provides detailed remediation guidance with code examples. Our scanner specifically targets vulnerabilities common in AI-generated applications.
Scans from $5, results in minutes. Get actionable results with step-by-step fix instructions tailored to your stack.
Get Starter ScanFrequently Asked Questions
Why can't React prevent all XSS?
React auto-escapes content in JSX: {userInput} is safe. BUT: dangerouslySetInnerHTML bypasses this protection, href='javascript:...' executes code, and React doesn't protect against DOM XSS from window.location or document.cookie manipulation. React reduces XSS risk significantly but doesn't eliminate it.
What's the difference between stored and reflected XSS?
Stored XSS: malicious script saved to database, executed when others view it (forum posts, profile bios). Reflected XSS: script in URL parameters, executed when victim clicks a crafted link. Stored XSS is typically more severe as it affects all users who view the content.
Is innerHTML always dangerous?
innerHTML with untrusted input is dangerous. Safe uses: innerHTML with hardcoded strings, or after sanitization with DOMPurify. NEVER: element.innerHTML = userInput. ALWAYS: element.textContent = userInput (for text), or DOMPurify.sanitize(userInput) before innerHTML.
How does CSP prevent XSS?
CSP blocks execution of scripts not explicitly allowed. With 'script-src self', injected inline scripts like <script>evil()</script> won't run because inline scripts aren't from 'self'. Even if XSS payload reaches the page, CSP stops execution. It's defense-in-depth, not a replacement for proper sanitization.