diff --git a/Gemfile b/Gemfile
index de23ef7d8..bb6973d4a 100644
--- a/Gemfile
+++ b/Gemfile
@@ -38,7 +38,7 @@ gem "sidekiq-unique-jobs"
 
 # Monitoring
 gem "vernier"
-gem "rack-mini-profiler"
+gem "rack-mini-profiler", group: :development
 gem "sentry-ruby"
 gem "sentry-rails"
 gem "sentry-sidekiq"
diff --git a/app/controllers/csp_reports_controller.rb b/app/controllers/csp_reports_controller.rb
new file mode 100644
index 000000000..47d4e722b
--- /dev/null
+++ b/app/controllers/csp_reports_controller.rb
@@ -0,0 +1,35 @@
+# Receives CSP violation reports from browsers.
+#
+# The Content Security Policy initializer sets `report_uri "/csp-violation-report"`.
+# Browsers POST a small JSON payload describing each blocked resource. We just log
+# the report — operators can forward these logs to Sentry or another aggregator.
+#
+# Inherits from ActionController::Base (not ApplicationController) to avoid auth,
+# CSRF, Pundit, and other before_actions. CSP reports must be accepted from any
+# origin, with no authentication, and the browser discards non-2xx responses.
+class CspReportsController < ActionController::Base
+  MAX_BODY_BYTES = 8_192
+
+  def create
+    body = request.body.read(MAX_BODY_BYTES)
+    report = parse_report(body)
+
+    Rails.logger.warn("[CSP] violation: #{report.to_json}") if report.present?
+
+    head :no_content
+  end
+
+  private
+    def parse_report(body)
+      return {} if body.blank?
+      JSON.parse(body)
+    rescue StandardError => e
+      # Catch broadly (JSON::ParserError, Encoding::CompatibilityError, etc.).
+      # Scrub to valid UTF-8 so String#truncate can't raise on malformed bytes,
+      # and fold the exception class into the log so a poorly-behaving client
+      # is still diagnosable without us 500-ing.
+      safe = body.to_s.dup.force_encoding(Encoding::UTF_8).scrub("")
+      Rails.logger.warn("[CSP] parse failed: #{e.class}") if defined?(Rails.logger)
+      { raw: safe.truncate(512), parse_error: e.class.name }
+    end
+end
diff --git a/config/environments/production.rb b/config/environments/production.rb
index fc7120a0a..b1649ca05 100644
--- a/config/environments/production.rb
+++ b/config/environments/production.rb
@@ -18,7 +18,7 @@
 
   # Ensures that a master key has been made available in ENV["RAILS_MASTER_KEY"], config/master.key, or an environment
   # key such as config/credentials/production.key. This key is used to decrypt credentials (and other encrypted files).
-  # config.require_master_key = true
+  config.require_master_key = true
 
   # Disable serving static files from `public/`, relying on NGINX/Apache to do so instead.
   # config.public_file_server.enabled = false
@@ -102,6 +102,26 @@
   config.active_record.dump_schema_after_migration = false
 
   # Enable DNS rebinding protection and other `Host` header attacks.
+  # Accept either APP_DOMAIN (single host) or ALLOWED_HOSTS (comma-separated list).
+  # If neither is set, `config.hosts` stays empty (allow all) to preserve backward
+  # compatibility with existing self-hosted deploys — a [SECURITY] warning is logged
+  # at boot (see cors.rb for the matching allow-list warning) to nudge operators.
+  if ENV["APP_DOMAIN"].present?
+    config.hosts = [ ENV["APP_DOMAIN"] ]
+  else
+    # Parse before deciding to assign — `ENV["ALLOWED_HOSTS"].present?` is
+    # true for junk like "," or "   " which split+strip+reject down to [].
+    # Assigning config.hosts = [] means "deny every host", which is worse
+    # than falling through to the warn-and-leave-unset branch.
+    parsed_hosts = ENV["ALLOWED_HOSTS"].to_s.split(",").map(&:strip).reject(&:empty?)
+    if parsed_hosts.any?
+      config.hosts = parsed_hosts
+    else
+      # Use `config.logger` rather than `Rails.logger`: this block runs during
+      # environment configuration, before `Rails.logger` is finalized.
+      config.logger&.warn("[SECURITY] APP_DOMAIN and ALLOWED_HOSTS not set — DNS rebinding protection disabled")
+    end
+  end
   # config.hosts = [
   #   "example.com",     # Allow requests from example.com
   #   /.*\.example\.com/ # Allow requests from subdomains like `www.example.com`
diff --git a/config/initializers/content_security_policy.rb b/config/initializers/content_security_policy.rb
index 834aa2118..99e3b1345 100644
--- a/config/initializers/content_security_policy.rb
+++ b/config/initializers/content_security_policy.rb
@@ -1,26 +1,91 @@
 # Be sure to restart your server when you modify this file.
-
-# Define an application-wide content security policy.
-# See the Securing Rails Applications Guide for more information:
-# https://guides.rubyonrails.org/security.html#content-security-policy-header
-
-# Rails.application.configure do
-#   config.content_security_policy do |policy|
-#     policy.default_src :self, :https
-#     policy.font_src    :self, :https, :data
-#     policy.img_src     :self, :https, :data
-#     policy.object_src  :none
-#     policy.script_src  :self, :https
-#     policy.script_src  :self, :https, 'https://us.i.posthog.com'
-#     policy.style_src   :self, :https
-#     # Specify URI for violation reports
-#     # policy.report_uri "/csp-violation-report-endpoint"
-#   end
 #
-#   # Generate session nonces for permitted importmap, inline scripts, and inline styles.
-#   config.content_security_policy_nonce_generator = ->(request) { request.session.id.to_s }
-#   config.content_security_policy_nonce_directives = %w(script-src style-src)
+# Content Security Policy — report-only mode.
+# Violations are logged to /csp-violation-report but nothing is blocked yet.
+# Once violations are reviewed and the policy is tuned, switch to enforcement
+# by removing config.content_security_policy_report_only = true.
 #
-#   # Report violations without enforcing the policy.
-#   # config.content_security_policy_report_only = true
-# end
+# Known external services that need allowlisting:
+#   - PostHog:  https://us.i.posthog.com, https://us-assets.i.posthog.com
+#   - Plaid:    https://cdn.plaid.com
+#   - Stripe:   https://js.stripe.com, https://hooks.stripe.com
+#   - Pusher:   wss://*.pusher.com (ActionCable / Hotwire)
+#   - OpenAI:   (server-side only, no browser resources needed)
+
+Rails.application.configure do
+  config.content_security_policy do |policy|
+    policy.default_src :self
+    policy.font_src    :self, :data
+    policy.img_src     :self, :https, :data, :blob
+    policy.object_src  :none
+
+    # Baseline hardening directives — all browsers respect these and they cost
+    # nothing: no arbitrary <base>, forms only to self, no framing.
+    policy.base_uri        :self
+    policy.form_action     :self
+    policy.frame_ancestors :none
+
+    managed_mode = Rails.application.config.app_mode.managed?
+
+    # Scripts: self + Plaid + Stripe (+ PostHog in managed mode) + importmap inline (nonce-controlled)
+    script_src = [
+      :self,
+      "https://cdn.plaid.com",
+      "https://js.stripe.com"
+    ]
+
+    # Connections: self + external APIs used client-side
+    connect_src = [
+      :self,
+      "https://hooks.stripe.com"
+    ]
+
+    if managed_mode
+      # Derive PostHog hosts from the same env var config/initializers/posthog.rb
+      # uses, so a managed deploy pointed at a different PostHog region doesn't
+      # have CSP silently blocking the very traffic that initializer enables.
+      #
+      # Region handling: the assets host mirrors the API host with the first
+      # subdomain labelled "-assets" (us.i.posthog.com → us-assets.i.posthog.com,
+      # eu.i.posthog.com → eu-assets.i.posthog.com). Operators on self-hosted
+      # PostHog with a different layout can override via POSTHOG_ASSETS_HOST.
+      posthog_host = ENV.fetch("POSTHOG_HOST", "https://us.i.posthog.com")
+      posthog_assets_host = ENV.fetch("POSTHOG_ASSETS_HOST") do
+        uri = URI.parse(posthog_host)
+        assets_host = uri.host.to_s.sub(/\A([^.]+)/, '\1-assets')
+        "#{uri.scheme}://#{assets_host}"
+      end
+
+      script_src += [ posthog_host, posthog_assets_host ]
+
+      connect_src += [
+        posthog_host,
+        posthog_assets_host,
+        "wss://*.pusher.com"
+      ]
+    end
+
+    policy.script_src(*script_src)
+
+    # Styles: self + unsafe_inline needed for Tailwind/inline styles
+    policy.style_src :self, :unsafe_inline
+
+    policy.connect_src(*connect_src)
+
+    # Frames: Plaid and Stripe use iframes
+    policy.frame_src "https://cdn.plaid.com",
+                     "https://js.stripe.com"
+
+    policy.report_uri "/csp-violation-report"
+  end
+
+  # Nonces for inline scripts/styles managed by importmap and Hotwire.
+  # Per-response random nonce — a session-id nonce is constant for the lifetime of
+  # the session and provides no CSP guarantee once an attacker observes one script tag.
+  config.content_security_policy_nonce_generator = ->(_request) { SecureRandom.base64(16) }
+  config.content_security_policy_nonce_directives = %w[script-src]
+
+  # REPORT-ONLY: violations are logged, nothing is blocked.
+  # Remove this line to enforce the policy.
+  config.content_security_policy_report_only = true
+end
diff --git a/config/initializers/cors.rb b/config/initializers/cors.rb
index 8ffb294d4..885cfa5de 100644
--- a/config/initializers/cors.rb
+++ b/config/initializers/cors.rb
@@ -2,35 +2,52 @@
 
 # CORS configuration for API access from mobile clients (Flutter) and other external apps.
 #
-# This enables Cross-Origin Resource Sharing for the /api, /oauth, and /sessions endpoints,
-# allowing the Flutter mobile client and other authorized clients to communicate
-# with the Rails backend.
+# Allowed origins configured via ALLOWED_ORIGINS env var (comma-separated).
+# Falls back to APP_DOMAIN if set, otherwise denies cross-origin requests.
+#
+# Examples:
+#   ALLOWED_ORIGINS=https://app.example.com,https://staging.example.com
+#   APP_DOMAIN=app.example.com
+#
+# Security: wildcard origins (*) are intentionally not used.
+
+def allowed_origins
+  if ENV["ALLOWED_ORIGINS"].present?
+    ENV["ALLOWED_ORIGINS"].split(",").map(&:strip).reject(&:empty?)
+  elsif ENV["APP_DOMAIN"].present?
+    [ "https://#{ENV['APP_DOMAIN']}" ]
+  else
+    Rails.logger.warn("[SECURITY] ALLOWED_ORIGINS and APP_DOMAIN not set — CORS will deny all cross-origin requests")
+    []
+  end
+end
 
 Rails.application.config.middleware.insert_before 0, Rack::Cors do
   allow do
-    # Allow requests from any origin for API endpoints
-    # Mobile apps and development environments need flexible CORS
-    origins "*"
+    origins(*allowed_origins)
 
-    # API endpoints for mobile client and third-party integrations
     resource "/api/*",
       headers: :any,
       methods: %i[get post put patch delete options head],
       expose: %w[X-Request-Id X-Runtime],
+      max_age: 86400,
+      credentials: true
+
+    resource "/oauth/token",
+      headers: :any,
+      methods: %i[post options],
       max_age: 86400
 
-    # OAuth endpoints for authentication flows
-    resource "/oauth/*",
+    resource "/oauth/revoke",
       headers: :any,
-      methods: %i[get post put patch delete options head],
-      expose: %w[X-Request-Id X-Runtime],
+      methods: %i[post options],
       max_age: 86400
 
-    # Session endpoints for webview-based authentication
     resource "/sessions/*",
       headers: :any,
       methods: %i[get post delete options head],
       expose: %w[X-Request-Id X-Runtime],
-      max_age: 86400
+      max_age: 86400,
+      credentials: true
   end
 end
diff --git a/config/initializers/mini_profiler.rb b/config/initializers/mini_profiler.rb
index 6a79f19ea..301677634 100644
--- a/config/initializers/mini_profiler.rb
+++ b/config/initializers/mini_profiler.rb
@@ -1,4 +1,7 @@
-Rails.application.configure do
-  Rack::MiniProfiler.config.skip_paths = [ "/design-system", "/assets", "/cable", "/manifest", "/favicon.ico", "/hotwire-livereload", "/logo-pwa.png" ]
-  Rack::MiniProfiler.config.max_traces_to_show = 50
+# rack-mini-profiler is only loaded in development
+if defined?(Rack::MiniProfiler)
+  Rails.application.configure do
+    Rack::MiniProfiler.config.skip_paths = [ "/design-system", "/assets", "/cable", "/manifest", "/favicon.ico", "/hotwire-livereload", "/logo-pwa.png" ]
+    Rack::MiniProfiler.config.max_traces_to_show = 50
+  end
 end
diff --git a/config/initializers/sidekiq.rb b/config/initializers/sidekiq.rb
index 1338491a4..91753f29e 100644
--- a/config/initializers/sidekiq.rb
+++ b/config/initializers/sidekiq.rb
@@ -1,12 +1,24 @@
 require "sidekiq/web"
 
 if Rails.env.production?
+  sidekiq_username = ENV["SIDEKIQ_WEB_USERNAME"]
+  sidekiq_password = ENV["SIDEKIQ_WEB_PASSWORD"]
+
+  if sidekiq_username.blank? || sidekiq_password.blank?
+    Rails.logger.warn("[SECURITY] SIDEKIQ_WEB_USERNAME and SIDEKIQ_WEB_PASSWORD not set — Sidekiq Web UI will be inaccessible")
+  end
+
   Sidekiq::Web.use(Rack::Auth::Basic) do |username, password|
-    configured_username = ::Digest::SHA256.hexdigest(ENV.fetch("SIDEKIQ_WEB_USERNAME", "sure"))
-    configured_password = ::Digest::SHA256.hexdigest(ENV.fetch("SIDEKIQ_WEB_PASSWORD", "sure"))
+    next false if sidekiq_username.blank? || sidekiq_password.blank?
 
-    ActiveSupport::SecurityUtils.secure_compare(::Digest::SHA256.hexdigest(username), configured_username) &
-      ActiveSupport::SecurityUtils.secure_compare(::Digest::SHA256.hexdigest(password), configured_password)
+    ActiveSupport::SecurityUtils.secure_compare(
+      ::Digest::SHA256.hexdigest(username),
+      ::Digest::SHA256.hexdigest(sidekiq_username)
+    ) &&
+    ActiveSupport::SecurityUtils.secure_compare(
+      ::Digest::SHA256.hexdigest(password),
+      ::Digest::SHA256.hexdigest(sidekiq_password)
+    )
   end
 end
 
diff --git a/config/routes.rb b/config/routes.rb
index 544d1c8f0..1a6a9206c 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -121,7 +121,7 @@
     delete :disable
   end
 
-  mount Lookbook::Engine, at: "/design-system"
+  mount Lookbook::Engine, at: "/design-system" if Rails.env.development?
 
   if Rails.env.development?
     mount Rswag::Api::Engine => "/api-docs"
@@ -538,6 +538,10 @@
   # MCP server endpoint for external AI assistants (JSON-RPC 2.0)
   post "mcp", to: "mcp#handle"
 
+  # Receive CSP violation reports from browsers (report_uri set in
+  # config/initializers/content_security_policy.rb).
+  post "csp-violation-report" => "csp_reports#create"
+
   # Reveal health status on /up that returns 200 if the app boots with no exceptions, otherwise 500.
   # Can be used by load balancers and uptime monitors to verify that the app is live.
   get "up" => "rails/health#show", as: :rails_health_check
diff --git a/docs/hosting/docker.md b/docs/hosting/docker.md
index 96ec28065..7146893b9 100644
--- a/docs/hosting/docker.md
+++ b/docs/hosting/docker.md
@@ -95,6 +95,20 @@ SECRET_KEY_BASE="replacemewiththegeneratedstringfromthepriorstep"
 POSTGRES_PASSWORD="replacemewithyourdesireddatabasepassword"
 ```
 
+#### Production hardening (recommended)
+
+The following variables tighten security when running outside a local network. Set them if your instance is reachable from the public internet.
+
+| Variable | Purpose |
+|---|---|
+| `RAILS_MASTER_KEY` | Decrypts Rails encrypted credentials. Required — the app refuses to boot in production without it (or a `config/master.key` file). |
+| `SIDEKIQ_WEB_USERNAME` + `SIDEKIQ_WEB_PASSWORD` | Basic-auth credentials guarding `/sidekiq`. The Web UI is mounted only when both are set. |
+| `ALLOWED_ORIGINS` | Comma-separated CORS allow-list for browser/mobile clients (e.g. `https://app.example.com,https://staging.example.com`). |
+| `APP_DOMAIN` | Single-host fallback for CORS when `ALLOWED_ORIGINS` is unset, and DNS-rebinding `config.hosts` allow-list. |
+| `ALLOWED_HOSTS` | Comma-separated alternative to `APP_DOMAIN` for DNS-rebinding protection when you serve multiple hostnames. |
+
+If none of `ALLOWED_ORIGINS` / `APP_DOMAIN` is set, cross-origin requests are rejected and a `[SECURITY]` warning is emitted at boot. Likewise if `APP_DOMAIN` / `ALLOWED_HOSTS` is missing, DNS-rebinding protection is disabled with a boot warning.
+
 #### Using HTTPS
 
 Assuming you want to access your instance from the internet, you should have secured your URL address with an SSL certificate.  
diff --git a/test/controllers/csp_reports_controller_test.rb b/test/controllers/csp_reports_controller_test.rb
new file mode 100644
index 000000000..8933ebf2f
--- /dev/null
+++ b/test/controllers/csp_reports_controller_test.rb
@@ -0,0 +1,50 @@
+require "test_helper"
+
+class CspReportsControllerTest < ActionDispatch::IntegrationTest
+  test "accepts a CSP report and returns 204" do
+    payload = { "csp-report" => { "violated-directive" => "script-src", "blocked-uri" => "https://evil.example.com/x.js" } }
+
+    post "/csp-violation-report",
+      params: payload.to_json,
+      headers: { "Content-Type" => "application/csp-report" }
+
+    assert_response :no_content
+  end
+
+  test "accepts an empty body and returns 204" do
+    post "/csp-violation-report"
+    assert_response :no_content
+  end
+
+  test "accepts malformed JSON without raising" do
+    post "/csp-violation-report",
+      params: "not-json",
+      headers: { "Content-Type" => "application/csp-report" }
+
+    assert_response :no_content
+  end
+
+  test "truncates bodies larger than MAX_BODY_BYTES without raising" do
+    # Send a valid-JSON payload whose byte length exceeds MAX_BODY_BYTES. The
+    # controller reads at most MAX_BODY_BYTES, producing a truncated (and
+    # therefore unparseable) body — must still return 204 and not 500.
+    oversized = "A" * (CspReportsController::MAX_BODY_BYTES + 1024)
+    payload = { "csp-report" => { "blocked-uri" => oversized } }.to_json
+
+    post "/csp-violation-report",
+      params: payload,
+      headers: { "Content-Type" => "application/csp-report" }
+
+    assert_response :no_content
+  end
+
+  test "accepts invalid-UTF-8 bytes without raising" do
+    # A hostile client can send bytes that make String#truncate blow up if we
+    # don't scrub. Must still return 204.
+    post "/csp-violation-report",
+      params: "\xC3\x28".b, # invalid 2-byte UTF-8 sequence
+      headers: { "Content-Type" => "application/csp-report" }
+
+    assert_response :no_content
+  end
+end
diff --git a/test/integration/cors_test.rb b/test/integration/cors_test.rb
index e16d98d3c..b4456f8ba 100644
--- a/test/integration/cors_test.rb
+++ b/test/integration/cors_test.rb
@@ -2,71 +2,84 @@
 
 require "test_helper"
 
+# CORS is now driven by ALLOWED_ORIGINS / APP_DOMAIN env vars (see F-02).
+# The test environment does not set either, so Rack::Cors is loaded with an
+# empty origin allowlist and MUST NOT echo back any Origin.
 class CorsTest < ActionDispatch::IntegrationTest
+  EVIL_ORIGIN = "http://evil.example.com"
+
+  def assert_cors_header_absent(msg = nil)
+    assert_nil response.headers["Access-Control-Allow-Origin"], msg
+  end
+
   test "rack cors is configured in middleware stack" do
     middleware_classes = Rails.application.middleware.map(&:klass)
     assert_includes middleware_classes, Rack::Cors, "Rack::Cors should be in middleware stack"
   end
 
-  test "cors headers are returned for api endpoints" do
-    get "/api/v1/usage", headers: { "Origin" => "http://localhost:3000" }
-
-    assert_equal "*", response.headers["Access-Control-Allow-Origin"]
-    assert response.headers["Access-Control-Expose-Headers"].present?
+  test "cors does not reflect wildcard origin for api endpoints" do
+    get "/api/v1/usage", headers: { "Origin" => EVIL_ORIGIN }
+    assert_cors_header_absent
   end
 
-  test "cors preflight request is handled for api endpoints" do
-    # Simulate a preflight OPTIONS request
+  test "cors preflight does not allow arbitrary origin for api endpoints" do
     options "/api/v1/transactions",
       headers: {
-        "Origin" => "http://localhost:3000",
+        "Origin" => EVIL_ORIGIN,
         "Access-Control-Request-Method" => "POST",
         "Access-Control-Request-Headers" => "Content-Type, Authorization"
       }
-
-    assert_response :ok
-    assert_equal "*", response.headers["Access-Control-Allow-Origin"]
-    assert response.headers["Access-Control-Allow-Methods"].present?
-    assert_includes response.headers["Access-Control-Allow-Methods"], "POST"
+    assert_cors_header_absent
   end
 
-  test "cors headers are returned for oauth endpoints" do
+  test "cors does not reflect wildcard origin for oauth token endpoint" do
     post "/oauth/token",
       params: { grant_type: "authorization_code", code: "test" },
-      headers: { "Origin" => "http://localhost:3000" }
-
-    assert_equal "*", response.headers["Access-Control-Allow-Origin"]
+      headers: { "Origin" => EVIL_ORIGIN }
+    assert_cors_header_absent
   end
 
-  test "cors preflight request is handled for oauth endpoints" do
+  test "cors preflight does not allow arbitrary origin for oauth token endpoint" do
     options "/oauth/token",
       headers: {
-        "Origin" => "http://localhost:3000",
+        "Origin" => EVIL_ORIGIN,
         "Access-Control-Request-Method" => "POST",
         "Access-Control-Request-Headers" => "Content-Type"
       }
+    assert_cors_header_absent
+  end
 
-    assert_response :ok
-    assert_equal "*", response.headers["Access-Control-Allow-Origin"]
+  test "cors does not reflect wildcard origin for oauth revoke endpoint" do
+    post "/oauth/revoke",
+      params: { token: "test-token" },
+      headers: { "Origin" => EVIL_ORIGIN }
+    assert_cors_header_absent
   end
 
-  test "cors headers are returned for session endpoints" do
+  test "cors preflight does not allow arbitrary origin for oauth revoke endpoint" do
+    options "/oauth/revoke",
+      headers: {
+        "Origin" => EVIL_ORIGIN,
+        "Access-Control-Request-Method" => "POST",
+        "Access-Control-Request-Headers" => "Content-Type"
+      }
+    assert_cors_header_absent
+  end
+
+  test "cors does not reflect wildcard origin for session endpoints" do
     post "/sessions",
       params: { email: "test@example.com", password: "password" },
-      headers: { "Origin" => "http://localhost:3000" }
-
-    assert_equal "*", response.headers["Access-Control-Allow-Origin"]
+      headers: { "Origin" => EVIL_ORIGIN }
+    assert_cors_header_absent
   end
 
-  test "cors preflight request is handled for session endpoints" do
+  test "cors preflight does not allow arbitrary origin for session endpoints" do
     options "/sessions/new",
       headers: {
-        "Origin" => "http://localhost:3000",
+        "Origin" => EVIL_ORIGIN,
         "Access-Control-Request-Method" => "GET",
         "Access-Control-Request-Headers" => "Content-Type"
       }
-
-    assert_response :ok
-    assert_equal "*", response.headers["Access-Control-Allow-Origin"]
+    assert_cors_header_absent
   end
 end