Matan · Security · Headers

CORS (ACAO)

Access-Control-Allow-Origin: * on a credentialed or sensitive response lets any origin read it.

Pass condition in our engine: Access-Control-Allow-Origin is not *.

1. Problem description

Wildcard ACAO is fine for truly public, cacheable APIs. It is not fine for sessions or personal data. We fail when ACAO is exactly *.

2. Most common causes

  • A CDN default for API routes.
  • A debug flag left on.

3. How to fix it

  1. Echo a specific origin instead of *.
  2. Never combine * with Access-Control-Allow-Credentials: true.
  3. Keep CORS off entirely for same-origin HTML pages.

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