diff --git a/app/controllers/concerns/active_storage_authentication.rb b/app/controllers/concerns/active_storage_authentication.rb
new file mode 100644
index 00000000..24065a3b
--- /dev/null
+++ b/app/controllers/concerns/active_storage_authentication.rb
@@ -0,0 +1,9 @@
+module ActiveStorageAuthentication
+  extend ActiveSupport::Concern
+  include Authentication::SessionLookup
+
+  private
+    def require_active_storage_authentication
+      head :unauthorized unless find_session_by_cookie
+    end
+end
diff --git a/config/initializers/active_storage_authentication.rb b/config/initializers/active_storage_authentication.rb
new file mode 100644
index 00000000..58eb91d8
--- /dev/null
+++ b/config/initializers/active_storage_authentication.rb
@@ -0,0 +1,16 @@
+# Active Storage mounts its direct-upload write endpoints
+# (POST /rails/active_storage/direct_uploads and the disk-service PUT) on
+# framework controllers that inherit from ActiveStorage::BaseController, so
+# they never pass through ApplicationController's require_authentication.
+# Writebook uploads attachments through ActionText::Markdown::UploadsController
+# as an ordinary multipart POST and does not use direct uploads at all, leaving
+# these endpoints reachable by anyone who can read a public page. Require a
+# valid Writebook session before an anonymous caller can allocate a Blob or
+# persist bytes to disk.
+Rails.application.config.to_prepare do
+  ActiveStorage::DirectUploadsController.include ActiveStorageAuthentication
+  ActiveStorage::DirectUploadsController.before_action :require_active_storage_authentication
+
+  ActiveStorage::DiskController.include ActiveStorageAuthentication
+  ActiveStorage::DiskController.before_action :require_active_storage_authentication, only: :update
+end
diff --git a/test/controllers/active_storage_authentication_test.rb b/test/controllers/active_storage_authentication_test.rb
new file mode 100644
index 00000000..a32596c3
--- /dev/null
+++ b/test/controllers/active_storage_authentication_test.rb
@@ -0,0 +1,80 @@
+require "test_helper"
+
+class ActiveStorageAuthenticationTest < ActionDispatch::IntegrationTest
+  test "direct upload metadata endpoint rejects anonymous callers" do
+    get new_session_path
+    assert_response :success
+
+    assert_no_difference -> { ActiveStorage::Blob.count } do
+      post rails_direct_uploads_path, params: blob_params, as: :json
+    end
+
+    assert_response :unauthorized
+  end
+
+  test "direct upload metadata endpoint allows authenticated users" do
+    sign_in :david
+
+    assert_difference -> { ActiveStorage::Blob.count }, 1 do
+      post rails_direct_uploads_path, params: blob_params, as: :json
+    end
+
+    assert_response :success
+  end
+
+  test "disk service upload endpoint rejects anonymous callers" do
+    sign_in :david
+    post rails_direct_uploads_path, params: blob_params, as: :json
+    assert_response :success
+    upload_path = URI.parse(response.parsed_body.dig("direct_upload", "url")).request_uri
+
+    anonymous = open_session
+    anonymous.put upload_path,
+      params: attachment_bytes,
+      headers: { "Content-Type" => "application/octet-stream" }
+
+    assert_equal 401, anonymous.status
+  end
+
+  test "disk service upload endpoint allows authenticated callers" do
+    sign_in :david
+    post rails_direct_uploads_path, params: blob_params, as: :json
+    assert_response :success
+    upload_path = URI.parse(response.parsed_body.dig("direct_upload", "url")).request_uri
+
+    put upload_path,
+      params: attachment_bytes,
+      headers: { "Content-Type" => "application/octet-stream" }
+
+    assert_response :no_content
+  end
+
+  test "disk service download endpoint stays public" do
+    ActiveStorage::Current.url_options = { host: "www.example.com", protocol: "https" }
+    blob = ActiveStorage::Blob.create_and_upload! \
+      io: StringIO.new(attachment_bytes), filename: "hi.txt", content_type: "text/plain"
+    download_path = URI.parse(blob.url).request_uri
+
+    anonymous = open_session
+    anonymous.get download_path
+
+    assert_equal 200, anonymous.status
+    assert_equal attachment_bytes, anonymous.response.body
+  ensure
+    blob&.purge
+  end
+
+  private
+    def attachment_bytes
+      "hello!"
+    end
+
+    def blob_params
+      { blob: {
+        filename: "quota.bin",
+        byte_size: attachment_bytes.bytesize,
+        checksum: Digest::MD5.base64digest(attachment_bytes),
+        content_type: "application/octet-stream"
+      } }
+    end
+end