TL;DR: Start with a threat model and secure defaults. Then learn authentication, authorization, input validation, output controls, rate limiting, secret management, logging, and incident response.
Threat Model First
List assets, actors, trust boundaries, abuse cases, and business impact. Security controls make more sense when each one addresses a concrete risk.
Protect the Request Lifecycle
Authenticate the caller, authorize the requested action against the resource, validate input, constrain output, and avoid leaking sensitive details through errors.
Add Abuse Controls
Use rate limits, pagination limits, request size limits, timeouts, idempotency keys, and audit logs. Test controls with invalid, repeated, and cross-tenant requests.
Verification Checklist
Review access control, token storage, password handling, CORS, dependency updates, secret rotation, logging redaction, and alerting. Security is an operating practice, not a middleware checkbox.