XSS Prevention Skill | Agent Skills

XSS Prevention

Overview

Implement comprehensive Cross-Site Scripting attack prevention through input sanitization, output encoding, Content Security Policy headers, and secure coding practices.

When to Use

User-generated content display
Rich text editors
Comment systems
Search functionality
Dynamic HTML generation
Template rendering scenarios

XSS Attack Types

| Type | Vector | Defense | |------|--------|---------| | Reflected | URL parameters | Output encoding | | Stored | Database content | Input sanitization | | DOM-based | Client-side JS | Safe DOM APIs | | Mutation | HTML parser quirks | Strict sanitization |

Output Encoding (Node.js)

function encodeHTML(str) {
  return str
    .replace(/&/g, '&amp;')
    .replace(/</g, '&lt;')
    .replace(/>/g, '&gt;')
    .replace(/"/g, '&quot;')
    .replace(/'/g, '&#x27;');
}

function encodeForAttribute(str) {
  return str.replace(/[^\w.-]/g, char =>
    `&#x${char.charCodeAt(0).toString(16)};`
  );
}

// Usage in templates
app.get('/profile', (req, res) => {
  const username = encodeHTML(req.query.name);
  res.send(`<h1>Welcome, ${username}</h1>`);
});

DOMPurify Sanitization

import DOMPurify from 'dompurify';

const config = {
  ALLOWED_TAGS: ['b', 'i', 'em', 'strong', 'a', 'p', 'br'],
  ALLOWED_ATTR: ['href', 'title'],
  ALLOW_DATA_ATTR: false
};

function sanitizeHTML(dirty) {
  return DOMPurify.sanitize(dirty, config);
}

// React component
function RichContent({ html }) {
  return (
    <div dangerouslySetInnerHTML={{ __html: sanitizeHTML(html) }} />
  );
}

Content Security Policy

// Express middleware
app.use((req, res, next) => {
  const nonce = crypto.randomBytes(16).toString('base64');
  res.locals.nonce = nonce;

  res.setHeader('Content-Security-Policy', [
    "default-src 'self'",
    `script-src 'self' 'nonce-${nonce}'`,
    "style-src 'self' 'unsafe-inline'",
    "img-src 'self' data: https:",
    "connect-src 'self' https://api.example.com",
    "frame-ancestors 'none'",
    "base-uri 'self'",
    "form-action 'self'"
  ].join('; '));

  next();
});

Safe DOM APIs

❌ NEVER do any of the following with user-controlled input — these are XSS sinks and there is no safe way to call them with untrusted data:

Assign it to element.innerHTML / element.outerHTML
Pass it to eval()
Pass it to document.write()
Pass it to setTimeout / setInterval as a string
Insert it into an inline event handler (e.g. onclick="...")

// SAFE — use these instead
element.textContent = userInput;      // Escaped automatically
element.setAttribute('data-id', id);  // Safe for attributes
document.createTextNode(userInput);   // Creates safe text node

The safe patterns above (textContent, attribute escaping via setAttribute, DOMPurify.sanitize) are the only correct ways to handle user input in the DOM.

URL Validation

function isSafeURL(url) {
  try {
    const parsed = new URL(url);
    return ['http:', 'https:'].includes(parsed.protocol);
  } catch {
    return false;
  }
}

// Usage
const href = isSafeURL(userURL) ? userURL : '#';

Context-Specific Encoding

Different contexts require different encoding approaches:

HTML Entity Encoding: Safest option for text content
Attribute Encoding: For HTML attributes
JavaScript Escaping: For script contexts
URL Encoding: For URL parameters
CSS Escaping: For stylesheet contexts

Always encode output by the specific context where data will be rendered.

Additional Implementations

See references/python-sanitization.md for:

Python bleach library usage
Flask/Django template escaping
Server-side validation patterns

See references/nodejs-advanced.md for:

Complete XSSPrevention class with all methods
Express middleware (xssProtection)
React components (SafeText, SafeHTML, SafeLink, useSanitizedInput)
Helmet CSP configuration

Best Practices

✅ DO:

Encode output by default
Use templating engines with auto-escaping
Implement CSP headers
Sanitize rich content with allowlists
Validate URLs with protocol whitelisting
Use HTTPOnly cookies
Conduct regular security testing
Leverage secure frameworks

❌ DON'T:

Trust user input
Use unsafe functions (eval, innerHTML)
Disable security features for convenience
Rely solely on client-side validation
Use blocklists instead of allowlists
Skip context-specific encoding
Allow arbitrary script execution

Security Checklist

[ ] Encode all output by context (HTML, attribute, JS)
[ ] Sanitize HTML with allowlist (not blocklist)
[ ] Implement strict CSP headers
[ ] Use HTTPOnly cookies for sessions
[ ] Validate and sanitize URLs
[ ] Avoid innerHTML with user content
[ ] Regular security testing

Agent Skills: XSS Prevention

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