fix: address Shannon security assessment findings (37 vulnerabilities) (#254)

Remediate 4 Critical, 18 High, 10 Medium, and 5 Low vulnerabilities
identified in the Shannon security assessment (2026-03-20).

Critical fixes:
- Command injection via rsync SSH key path (INJ-VULN-01)
- Self-escalation to super_admin role (AUTHZ-VULN-11)
- Invite super_admin backdoor (AUTHZ-VULN-12)
- Handlebars SSTI in email templates (INJ-VULN-05)

Authentication hardening:
- Rate limit on share-link login (AUTH-VULN-01)
- X-Forwarded-For spoofing bypass (AUTH-VULN-02)
- reCAPTCHA fails closed when misconfigured (AUTH-VULN-03)
- Token revocation on admin/gallery logout (AUTH-VULN-04/05)
- Cookie Secure flag defaults true in production (AUTH-VULN-06)
- Remove JWT from admin login response body (AUTH-VULN-07)
- Timing-safe gallery slug validation (AUTH-VULN-09)
- Account lockout fails closed on DB error (AUTH-VULN-12)
- Session endpoint checks token revocation

Path traversal & file access:
- checksums endpoint path containment (INJ-VULN-03)
- manifest validate path containment (INJ-VULN-04)

XSS prevention:
- Block SVG data URIs in CSS sanitizer (XSS-VULN-01)
- Email preview iframe sandbox (XSS-VULN-02)
- SSR branding HTML escaping (XSS-VULN-03)
- User-Agent sanitization in feedback (XSS-VULN-04)

Authorization (IDOR):
- Event ownership middleware for all admin routes
- Cross-admin user profile read restriction (AUTHZ-VULN-10)

SSRF & infrastructure:
- Private IP validation for SMTP, S3, rsync hosts
- Replace inline JWT with standard adminAuth middleware
- CSRF Content-Type enforcement on mutating API endpoints
- CSP headers in nginx location blocks

Token revocation fix:
- Remove overly broad orWhere clause that invalidated all future tokens
- Allow empty-body POST requests (logout) in CSRF middleware

Co-authored-by: Paul Nothaft <[email protected]>

backend/server.js

@@ -194,13 +194,23 @@ function composeInlineStyles(payload) {
   return cssSegments.join('\n\n');
 }
 
+function escapeHtml(str) {
+  if (!str) return '';
+  return String(str)
+    .replace(/&/g, '&')
+    .replace(/</g, '&lt;')
+    .replace(/>/g, '&gt;')
+    .replace(/"/g, '&quot;')
+    .replace(/'/g, '&#039;');
+}
+
 function renderBrandHeader(branding) {
-  const displayName = branding.companyName || 'PicPeak';
-  const logoSrc = branding.logoUrl || '/picpeak-logo-transparent.png';
+  const displayName = escapeHtml(branding.companyName || 'PicPeak');
+  const logoSrc = encodeURI(branding.logoUrl || '/picpeak-logo-transparent.png');
   const logo = `<img src="${logoSrc}" alt="${displayName}" class="brand-logo" loading="lazy" decoding="async" />`;
 
   const tagline = branding.companyTagline
-    ? `<p class="brand-tagline">${branding.companyTagline}</p>`
+    ? `<p class="brand-tagline">${escapeHtml(branding.companyTagline)}</p>`
     : '';
 
   return `<header class="site-header">
@@ -224,13 +234,14 @@ function renderBrandHeader(branding) {
 }
 
 function renderBrandFooter(branding) {
-  const displayName = branding.companyName || 'PicPeak';
+  const displayName = escapeHtml(branding.companyName || 'PicPeak');
   const footerNote = branding.footerText
-    ? `<p>${branding.footerText}</p>`
+    ? `<p>${escapeHtml(branding.footerText)}</p>`
     : '<p>Powered by PicPeak to keep every celebration beautifully organised.</p>';
 
-  const supportLink = branding.supportEmail
-    ? `<a href="mailto:${branding.supportEmail}">Support</a>`
+  const supportEmail = escapeHtml(branding.supportEmail || '');
+  const supportLink = supportEmail
+    ? `<a href="mailto:${supportEmail}">Support</a>`
     : '';
 
   const legalLinks = `
@@ -282,7 +293,7 @@ function buildPublicSiteDocument(payload) {
   <meta charset="utf-8" />
   <meta http-equiv="X-UA-Compatible" content="IE=edge" />
   <meta name="viewport" content="width=device-width, initial-scale=1" />
-  <title>${payload.title}</title>
+  <title>${escapeHtml(payload.title)}</title>
   <meta name="description" content="Curated photo galleries and stories from unforgettable celebrations." />
   ${seoMeta}
   <link rel="preconnect" href="https://fonts.googleapis.com" />
@@ -362,6 +373,20 @@ async function initializeRateLimiters() {
 app.use(express.json({ limit: '50mb' }));
 app.use(express.urlencoded({ extended: true, limit: '50mb' }));
 
+// CSRF protection: require JSON Content-Type on mutating API requests
+// This blocks cross-origin form submissions which cannot set Content-Type: application/json
+app.use('/api', (req, res, next) => {
+  if (['POST', 'PUT', 'DELETE', 'PATCH'].includes(req.method)) {
+    const contentType = req.headers['content-type'] || '';
+    const contentLength = parseInt(req.headers['content-length'] || '0', 10);
+    // Allow empty-body requests (e.g. logout), multipart for uploads, and JSON for API calls
+    if (contentLength > 0 && !contentType.includes('application/json') && !contentType.includes('multipart/form-data')) {
+      return res.status(415).json({ error: 'Unsupported Content-Type. Use application/json or multipart/form-data.' });
+    }
+  }
+  next();
+});
+
 // Request logging for API routes (with timestamps)
 const apiRequestLogger = (req, res, next) => {
   try {