Firebase Security Rules: First Steps That Prevent Expensive Mistakes
Start locked-down by default for Firestore and Storage — authentication checks, least-privilege reads, and validation patterns beginners overlook.
Table of Contents
Firebase Security Rules: First Steps That Prevent Expensive Mistakes
Security rules are not an afterthought retrofit — they are your database ACL for client SDKs. Begin from deny-by-default, then open precise paths as product requirements clarify.
Mental model
Rules evaluate every SDK request — browsers cannot bypass them. Treat anything shipped to web or mobile clients as public eventually; assume curious users inspect bundles.
Pattern checklist
- Require
request.auth != nullfor private collections unless data is intentionally world-readable marketing copy. - Scope document reads with ownership predicates (
resource.data.ownerId == request.auth.uid). - Validate shape on writes — reject unexpected fields early instead of debugging corrupt rows later.
Storage specifics
Use path-level constraints (request.auth.uid == userId) and content-type/size checks when uploads should stay bounded.
Testing discipline
Use the emulator suite to replay golden-path and adversarial requests before shipping rule changes — regressions are cheaper locally than in production incident reviews.
Iterate rules alongside schema evolution — outdated comments lying about behaviour are a liability.
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