CORS for Next.js
medium4 checks
CORS decides which origins may read your API responses. A wildcard turns an authenticated API into a public one.
What Next.js must have
- Access-Control-Allow-Origin limited to known origins
- No wildcard combined with credentials
- No reflection of arbitrary Origin headers
- Preflight responses only allow the methods actually used
How to fix it
- 1Replace wildcards with an explicit allowlist
- 2Validate Origin before echoing it and reject unknown origins early
Next.js context
React framework with server-side rendering, routing and API routes, usually deployed on Vercel or Node.
Server components and middleware carry the auth boundary. Middleware-only auth was bypassable via CVE-2025-29927 — enforce authorisation in the route itself.