CORS for Express
medium4 checks
CORS decides which origins may read your API responses. A wildcard turns an authenticated API into a public one.
What Express 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
Express context
Node.js HTTP framework for APIs and server-rendered apps.
Disable x-powered-by, set security headers via helmet, and validate body/query input to avoid prototype pollution.