Rails Security Multitenancy

Rails Security + Multi-Tenancy

Use for security-sensitive Rails work and tenant-boundary reviews. Patterns from Fizzy (path-based multi-tenant SaaS) and Campfire (single-tenant, bot APIs).

Core Rules

• Scope all tenant data access through tenant/user ownership boundaries.
• Never trust naked Model.find(params[:id]) in tenant-aware flows.
• Scope realtime broadcasts and stream names by tenant/account.
• Rate-limit auth and abuse-prone endpoints.
• Treat user-provided URLs as untrusted input.
• Fail closed (head :forbidden) when access cannot be proven.

Tenancy Architecture (path-based, Fizzy-style)

• Middleware extracts the account prefix from PATH_INFO into SCRIPT_NAME, loads the account, and wraps the request in Current.with_account(account). URL helpers stay tenant-correct automatically (including ActiveStorage and webhook payload URLs via script_name:).
• Three-layer auth: Identity → Session → account-scoped User. Sessions attach to an Identity; each request resolves Current.user = identity.users.find_by(account: Current.account). A valid session in account A can never act in account B. Apply the same resolution in ActionCable Connection#connect.
• Current setters cascade: assigning session resolves identity, assigning identity resolves user for the current account.
• Auth routes (login, signup, magic links) explicitly opt out of tenancy (disallow_account_scope) and redirect away from tenant-prefixed URLs.
• Path-scope session cookies (path: account.slug) when simultaneous multi-tenant login is supported.
• Recurring jobs run outside request context: iterate tenants explicitly (with_each_tenant). Serialize Current.account into job payloads (see rails-jobs).
• Test the tenancy middleware in isolation with Rack::MockRequest; integration tests set default_url_options[:script_name] from the fixture account.

Scoped Lookups (defense in depth)

Params choose which record within an already-authorized set — never establish access:

@card = Current.user.accessible_cards.find_by!(number: params[:card_id])   # access graph
@membership = Current.user.memberships.find_by!(room_id: params[:room_id]) # join model
@user = Current.account.users.find(params[:user_id])                       # tenant association

• Even single-tenant code scopes through associations; wrong IDs 404 naturally.
• Public sharing uses opaque tokens (has_secure_token :key on a Publication record), never internal IDs.
• ActiveStorage: attach blobs to accounts, and authorize blob/representation controllers through the domain (blob → attachment → record.accessible_to?(user)); published content gets an explicit publicly_accessible? path.
• Revoking access cleans up derived data (mentions, notifications, watches) via a scoped async job — don't leave dangling cross-boundary state.

Authentication Hardening

• Magic links / codes: single-use (consume destroys the row), short-lived, compared with secure_compare, bound to the email via a verified pending-auth cookie.
• Anti-enumeration: unknown email gets the same fake flow/UX as a real one.
• API tokens: HTTP-method-scoped permissions (read-only tokens can't POST); show generated secrets once via a short-lived message verifier (~10s), then never again.
• Bot/automation auth as an explicit mode: skip CSRF only for bot-key auth, deny bots everywhere by default (deny_bots + allow_bot_access only:).
• Rate limit with Rails built-ins, with responses matching endpoint semantics:

rate_limit to: 10, within: 15.minutes, only: :create,
  with: -> { redirect_to ..., alert: "Try again in 15 minutes." }

• Throttle session bookkeeping writes (update last_active_at at most hourly).
• Filter sensitive params beyond passwords: message bodies, push endpoints, tokens.

SSRF Defense Baseline

For webhooks, push endpoints, unfurling — any user-influenced URL:

• Resolve DNS and validate the destination IP before the request; block loopback/private/link-local/IPv4-mapped-IPv6 ranges (link-local = cloud metadata).
• Pin the request to the validated IP (Net::HTTP.new(host, port, ipaddr: resolved_ip)) to beat DNS rebinding.
• Validate at creation time and again at execution time.
• Re-resolve and re-validate on every redirect hop — redirect chains are the classic bypass.
• Cap response sizes (content-length pre-check + chunked read limit) to prevent memory DoS.
• Layer allowlists on top where the destination set is known (web push: permitted vendor host suffixes AND public-IP resolution).

CSRF and Caching

• Never HTTP-cache pages that render forms/CSRF tokens.
• Add Sec-Fetch-Site verification (same-origin/same-site) on top of origin checks; roll out in report-only mode first, and append Sec-Fetch-Site to the Vary header.
• Set a CSP with a hard floor (object_src :none, base_uri :none, frame_ancestors :self); use report-only + report_uri to validate before enforcing.
• Private apps: send X-Robots-Tag: none.

Authorization Defaults

• Predicate methods on models (card.editable_by?(user), user.can_administer_board?(board)); controllers check and head :forbidden.
• Declarative controller macros (allow_unauthenticated_access, ensure_can_administer) over scattered conditionals.
• Centralize shared guard logic in small concerns.

Abuse Response

• Banning a user cascades: convert their session IPs to bans, disconnect ActionCable remotely, delete sessions, purge content async with UI broadcasts.
• Enforce IP bans only on mutating requests (POST/PUT/PATCH/DELETE).
• Validate ban targets are public IPs — never let loopback/private ranges be banned.
• Disconnect deactivated users: ActionCable.server.remote_connections.where(current_user: user).disconnect(reconnect: false).

Inbound Webhooks (receiving)

• Verify signatures (e.g. Stripe::Webhook.construct_event), then re-fetch canonical state from the source API rather than trusting payload contents/ordering.

Red Flags

• Tenant-unscoped broadcast channels or stream names.
• Tenant data inferred from request params without ownership checks.
• A global User looked up by session without account scoping (identity/user conflation).
• Webhook or fetch requests to unvalidated destinations; redirects followed without re-validation.
• ActiveStorage URLs that bypass domain authorization.
• Security controls hidden in ad-hoc conditionals across controllers.
• Secrets permanently visible in admin UIs.
• Auth endpoints without rate limiting.