Harden auth, payments, and frontend against review findings.

Close exploitable gaps in booking/payment flows, enforce token versioning and account checks, gate sensitive payment data, and add middleware plus input validation across admin routes.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/lib/emailTemplates.ts

@@ -1216,8 +1216,26 @@ Spanglish`,
   },
 ];
 
-// Helper function to replace template variables
-export function replaceTemplateVariables(template: string, variables: Record<string, any>): string {
+// Escape HTML-significant characters so substituted variable values can't inject markup.
+function escapeHtmlValue(value: string): string {
+  return value
+    .replace(/&/g, '&amp;')
+    .replace(/</g, '&lt;')
+    .replace(/>/g, '&gt;')
+    .replace(/"/g, '&quot;')
+    .replace(/'/g, '&#x27;');
+}
+
+// Helper function to replace template variables.
+// When `escapeHtml` is true (HTML rendering contexts), substituted *values* are
+// HTML-escaped to prevent stored-XSS via attacker-influenced variables (e.g. event
+// location, attendee name). The template markup itself is never escaped. Subjects and
+// plain-text bodies pass `escapeHtml=false` so they don't show literal entities.
+export function replaceTemplateVariables(
+  template: string,
+  variables: Record<string, any>,
+  escapeHtml: boolean = false
+): string {
   let result = template;
   
   // Handle conditional blocks {{#if variable}}...{{/if}}
@@ -1229,16 +1247,20 @@ export function replaceTemplateVariables(template: string, variables: Record<str
   // Replace simple variables {{variable}}
   const variableRegex = /\{\{(\w+)\}\}/g;
   result = result.replace(variableRegex, (match, varName) => {
-    return variables[varName] !== undefined ? String(variables[varName]) : match;
+    if (variables[varName] === undefined) return match;
+    const raw = String(variables[varName]);
+    return escapeHtml ? escapeHtmlValue(raw) : raw;
   });
   
   return result;
 }
 
-// Helper to wrap content in the base template
+// Helper to wrap content in the base template.
+// `content` is treated as trusted HTML (it is the already-rendered body). The wrapper's
+// own variables (subject, year, etc.) are HTML-escaped on substitution.
 export function wrapInBaseTemplate(content: string, variables: Record<string, any>): string {
-  const wrappedContent = baseEmailWrapper.replace('{{content}}', content);
-  return replaceTemplateVariables(wrappedContent, variables);
+  const wrappedContent = baseEmailWrapper.replace('{{content}}', () => content);
+  return replaceTemplateVariables(wrappedContent, variables, true);
 }
 
 // Get all available variables for a template by slug