Security Rules are Firebase policies that decide which authenticated or anonymous users can read, write, or update specific data. They act as the access-control layer for a cloud-backed app, often checking the user’s identity, request path, and data shape before allowing an operation. In practice, these rules are the real boundary between tenants, projects, and private records.
They matter because a loose rule can expose far more than the app interface suggests. If a query is too broad, or a rule fails to enforce ownership correctly, one user may see another user’s documents, metadata, or meeting records. Attackers look for these mistakes because they often require no malware at all—just normal requests that the backend wrongly accepts. Defenders use deny-by-default rules, emulator testing, and careful review of read/write conditions to prevent accidental data exposure.