CORS for Firebase
medium4 checks
CORS decides which origins may read your API responses. A wildcard turns an authenticated API into a public one.
What Firebase 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
Firebase context
Google backend platform (Firestore, Auth, Storage) used by web and mobile apps.
Security rules must be verified; open Firestore/Storage rules are a classic data-leak finding.