# CORS + XSS Security Implementation Summary

## Overview

This document summarizes the security enhancements implemented to address CORS and XSS vulnerabilities (Issue #14 from the Security Audit Report).

## Implementation Date

2024

## Status

✅ **FULLY IMPLEMENTED**

## Changes Made

### 1. CORS Startup Validation

**File:** `src/utils/corsValidator.js`

- **Startup Validation:** Validates CORS configuration when the application starts
- **Production Enforcement:** Fails fast if CORS origins are not configured in production
- **Origin Format Validation:** Checks for valid URL format and warns about suspicious patterns
- **Wildcard Detection:** Prevents wildcard (`*`) usage in production

**Features:**
- Validates that `CORS_ALLOWED_ORIGINS` is set in production
- Checks origin format (must be valid URLs)
- Warns about HTTP origins in production (should be HTTPS)
- Prevents wildcard origins in production
- Provides clear error messages for misconfiguration

### 2. Runtime CORS Checks

**File:** `src/utils/corsValidator.js`

- **Runtime Monitoring:** Checks CORS requests at runtime
- **Suspicious Pattern Detection:** Identifies potentially misconfigured origins
- **Logging:** Logs warnings for suspicious CORS patterns

**Features:**
- Detects HTTP origins in production
- Identifies localhost/127.0.0.1 usage in production
- Logs blocked origins for monitoring
- Provides runtime feedback without blocking legitimate requests

### 3. Content Security Policy (CSP)

**File:** `src/middleware/securityHeaders.js`

- **CSP Headers:** Implements comprehensive Content Security Policy
- **Nonce Support:** Generates unique nonces for each request to allow safe inline scripts/styles
- **Strict Directives:** Restricts resource loading to prevent XSS attacks

**CSP Directives:**
- `default-src 'self'` - Only allow resources from same origin
- `script-src 'self' 'nonce-...' 'unsafe-eval'` - Scripts from self or with nonce
- `style-src 'self' 'nonce-...'` - Styles from self or with nonce
- `img-src 'self' data: https:` - Images from self, data URIs, or HTTPS
- `font-src 'self' data: https:` - Fonts from self, data URIs, or HTTPS
- `connect-src 'self' [CORS origins]` - API calls to self or allowed CORS origins
- `frame-ancestors 'none'` - Prevent embedding (clickjacking protection)
- `base-uri 'self'` - Restrict base tag
- `form-action 'self'` - Forms can only submit to same origin
- `upgrade-insecure-requests` - Upgrade HTTP to HTTPS

**Nonce Usage:**
- Nonce is generated per request and stored in `res.locals.cspNonce`
- Can be used in templates/views for inline scripts/styles
- Example: `<script nonce="<%= res.locals.cspNonce %>">...</script>`

### 4. Additional Security Headers

**File:** `src/middleware/securityHeaders.js`

Added headers:
- **Referrer-Policy:** `strict-origin-when-cross-origin` - Controls referrer information
- **Permissions-Policy:** Restricts browser features (geolocation, camera, microphone, etc.)

Existing headers maintained:
- `X-Frame-Options: DENY` - Clickjacking protection
- `X-Content-Type-Options: nosniff` - MIME type sniffing protection
- `X-XSS-Protection: 1; mode=block` - Legacy XSS filter
- `Strict-Transport-Security` - HSTS for HTTPS enforcement

### 5. Global Security Headers Application

**File:** `src/index.js`

- **Before:** Security headers only applied to admin routes
- **After:** Security headers applied to ALL routes globally
- **Placement:** Applied early in middleware chain for maximum protection

### 6. XSS Prevention Documentation

**File:** `XSS_PREVENTION_GUIDE.md`

Comprehensive guide covering:
- Server-side protections
- Frontend best practices
- Output encoding techniques
- Framework-specific guidance (React, Vue, Angular)
- Common attack vectors
- Testing methodologies
- Security checklist

## Configuration

### Environment Variables

No new environment variables required. Uses existing:
- `CORS_ALLOWED_ORIGINS` - Comma-separated list of allowed origins (required in production)
- `NODE_ENV` - Set to `production` for strict validation

### CSP Configuration

CSP is automatically configured. To customize:

1. Edit `src/middleware/securityHeaders.js`
2. Modify the `buildCSP()` function
3. Adjust directives as needed

**Note:** Current CSP allows `'unsafe-inline'` and `'unsafe-eval'` for compatibility. Consider tightening in production by:
- Removing `'unsafe-inline'` and using nonces exclusively
- Removing `'unsafe-eval'` if not needed

## Testing

### CORS Validation Testing

1. **Test startup validation:**
   ```bash
   # Should fail in production without CORS_ALLOWED_ORIGINS
   NODE_ENV=production node src/index.js
   ```

2. **Test runtime checks:**
   - Make request from non-whitelisted origin
   - Check logs for warnings

### CSP Testing

1. **Test CSP headers:**
   ```bash
   curl -I http://localhost:3000/health
   # Check for Content-Security-Policy header
   ```

2. **Test nonce generation:**
   - Each request should have unique nonce
   - Check `res.locals.cspNonce` in route handlers

### Browser Testing

1. Open browser DevTools → Network tab
2. Check response headers for:
   - `Content-Security-Policy`
   - `X-Frame-Options`
   - `Referrer-Policy`
   - `Permissions-Policy`

## Security Impact

### Before
- ⚠️ No CORS validation at startup
- ⚠️ No runtime CORS monitoring
- ⚠️ No CSP headers
- ⚠️ Security headers only on admin routes
- ⚠️ No XSS prevention guidance

### After
- ✅ CORS validated at startup (fails fast if misconfigured)
- ✅ Runtime CORS monitoring and logging
- ✅ Comprehensive CSP with nonce support
- ✅ Security headers on all routes
- ✅ Complete XSS prevention documentation

## Risk Level

**Before:** 🟡 MEDIUM  
**After:** 🟢 LOW

## Migration Notes

### Breaking Changes

None. All changes are backward compatible.

### Recommendations

1. **Tighten CSP in production:**
   - Remove `'unsafe-inline'` and use nonces
   - Remove `'unsafe-eval'` if not needed

2. **Monitor CORS logs:**
   - Watch for blocked origins
   - Review suspicious pattern warnings

3. **Update frontend code:**
   - Follow XSS prevention guide
   - Use nonces for inline scripts/styles
   - Sanitize user input

## Files Modified

1. `src/utils/corsValidator.js` - **NEW FILE**
2. `src/middleware/securityHeaders.js` - **ENHANCED**
3. `src/index.js` - **UPDATED**
4. `XSS_PREVENTION_GUIDE.md` - **NEW FILE**
5. `SECURITY_AUDIT_REPORT.md` - **UPDATED**

## References

- [OWASP XSS Prevention Cheat Sheet](https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html)
- [MDN Content Security Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP)
- [CORS Specification](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS)

## Support

For questions or issues:
1. Review `XSS_PREVENTION_GUIDE.md` for frontend guidance
2. Check `SECURITY_AUDIT_REPORT.md` for security context
3. Review code comments in implementation files

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**Implementation Status:** ✅ Complete  
**Testing Status:** ✅ Syntax validated  
**Documentation Status:** ✅ Complete
