Resolving Common CORS and Domain Matching Issues in Next.js Auth

Cross-Origin Resource Sharing (CORS) headaches often pop up when your frontend application is hosted on one subdomain (e.g., app.domain.com) while your authentication server or API routes live on another (e.g., api.domain.com).

Properly configuring cookie domains (domain: '.domain.com') and setting explicit CORS headers in your Next.js middleware or Express backend solves 90% of cookie-sharing issues across subdomains.