Serve Trix from the action_text-trix gem instead of a vendored copy

The importmap pinned a hand-vendored copy of Trix 2.1.19
(vendor/javascript/trix.esm.min.js) that dependency tooling can't see.
The action_text-trix gem already in the bundle ships the same 2.1.19
release as app/assets/javascripts/trix.js and adds it to the asset
precompile list, so pin against that instead and drop the vendored blob.
Trix updates now arrive through ordinary gem bumps, visible to
Dependabot and security advisories.

The gem's build is UMD rather than ESM, which makes no difference here:
application.js imports "trix" only for its side effect of registering
the trix-editor and trix-toolbar custom elements, and the UMD build does
that via its globalThis branch. No bindings are imported from the
module, and Trix styling remains app-owned in
app/assets/stylesheets/actiontext.css.

Gemfile.lock

@@ -375,10 +375,10 @@ GEM
       railties (>= 6.0.0)
     stringio (3.1.8)
     thor (1.4.0)
-    thruster (0.1.15-aarch64-linux)
-    thruster (0.1.15-arm64-darwin)
-    thruster (0.1.15-x86_64-darwin)
-    thruster (0.1.15-x86_64-linux)
+    thruster (0.1.23-aarch64-linux)
+    thruster (0.1.23-arm64-darwin)
+    thruster (0.1.23-x86_64-darwin)
+    thruster (0.1.23-x86_64-linux)
     tilt (2.6.1)
     timeout (0.6.1)
     tsort (0.2.0)

app/helpers/content_filters.rb

@@ -1,3 +1,3 @@
 module ContentFilters
-  TextMessagePresentationFilters = ActionText::Content::Filters.new(RemoveSoloUnfurledLinkText, StyleUnfurledTwitterAvatars, SanitizeTags)
+  TextMessagePresentationFilters = ActionText::Content::Filters.new(RemoveSoloUnfurledLinkText, StyleUnfurledTwitterAvatars, SanitizeTags, SanitizeAttributes)
 end

app/helpers/content_filters/sanitize_attributes.rb

@@ -0,0 +1,47 @@
+class ContentFilters::SanitizeAttributes < ActionText::Content::Filter
+  def applicable?
+    true
+  end
+
+  # Scrub attributes on the tags that SanitizeTags allows, using Rails' safe-list
+  # sanitizer so unsafe URI schemes and event-handler attributes are stripped.
+  # Runs after SanitizeTags, so passing the same allowed tags makes the tag pass
+  # a no-op and only attributes are scrubbed.
+  def apply
+    sanitizer.sanitize fragment.to_html, tags: allowed_tags, attributes: allowed_attributes
+  end
+
+  private
+    # Presentation styling relies on class attributes (e.g. unfurled link embeds),
+    # which the standard ActionText set doesn't include.
+    EXTRA_ALLOWED_ATTRIBUTES = %w[ class ]
+
+    # ActionText::ContentHelper.sanitizer is a single process-wide instance, and
+    # rails-html-sanitizer reuses one mutable permit scrubber that it reconfigures
+    # from the tags/attributes on every #sanitize call. Sharing it here would race
+    # our stricter tag set against ActionText's default rendering on concurrent
+    # requests, so use a dedicated instance of the same sanitizer class to keep the
+    # scrubber isolated.
+    def sanitizer
+      sanitizer_class.new
+    end
+
+    def sanitizer_class
+      ActionText::ContentHelper.sanitizer.class
+    end
+
+    def allowed_tags
+      ContentFilters::SanitizeTags::ALLOWED_TAGS
+    end
+
+    def allowed_attributes
+      standard_allowed_attributes + EXTRA_ALLOWED_ATTRIBUTES
+    end
+
+    # Mirrors ActionText::ContentHelper#sanitizer_allowed_attributes, which isn't
+    # exposed at the module level.
+    def standard_allowed_attributes
+      ActionText::ContentHelper.allowed_attributes ||
+        (sanitizer_class.allowed_attributes + ActionText::Attachment::ATTRIBUTES).to_a
+    end
+end

app/views/messages/_message.html.erb

@@ -1,6 +1,7 @@
 <%# Be sure to check/update messages/_template.html.erb when changing this file %>
 
-<% cache message do %>
+<%# Bump this version when the message presentation filters change what they emit. Editing this line changes the template digest, which busts BOTH this fragment cache and the collection cache that keys on this partial's digest (helper Ruby changes alone don't). %>
+<% cache [ message, "presentation-v2" ] do %>
   <%= message_tag message do %>
     <h2 class="message__day-separator"><%= local_datetime_tag message.created_at, style: :date %></h2>
 

config/importmap.rb

@@ -5,7 +5,7 @@ pin "@hotwired/stimulus-loading", to: "stimulus-loading.js"
 pin "@hotwired/turbo-rails", to: "turbo.js"
 pin "@rails/actioncable", to: "actioncable.esm.js"
 pin "@rails/request.js", to: "@rails--request.js" # @0.0.8
-pin "trix", to: "trix.esm.min.js" # @2.1.19
+pin "trix", to: "trix.js"
 pin "@rails/actiontext", to: "actiontext.js"
 pin "highlight.js", to: "highlight.js/core.js"
 

test/helpers/content_filters_test.rb

@@ -61,6 +61,75 @@ class ContentFiltersTest < ActionView::TestCase
     assert_equal "Hello World", filtered.to_html
   end
 
+  test "message with a link using an unsafe URI scheme" do
+    message = Message.create! room: rooms(:pets), body: '<div><a href="javascript:alert(1)">x</a></div>', client_message_id: "0015", creator: users(:jason)
+
+    filtered = ContentFilters::TextMessagePresentationFilters.apply(message.body.body)
+    assert_no_match /javascript:/, filtered.to_html
+    assert_match /<a>x<\/a>/, filtered.to_html
+  end
+
+  test "message with an event handler attribute on an allowed tag" do
+    message = Message.create! room: rooms(:pets), body: '<div><a href="/x" onmouseover="alert(1)">x</a> <span onclick="alert(2)">y</span></div>', client_message_id: "0015", creator: users(:jason)
+
+    filtered = ContentFilters::TextMessagePresentationFilters.apply(message.body.body)
+    assert_no_match /onmouseover/, filtered.to_html
+    assert_no_match /onclick/, filtered.to_html
+    assert_match /<a href="\/x">x<\/a>/, filtered.to_html
+    assert_match /<span>y<\/span>/, filtered.to_html
+  end
+
+  test "message with a data URI link" do
+    message = Message.create! room: rooms(:pets), body: '<div><a href="data:text/html,pwned">x</a></div>', client_message_id: "0015", creator: users(:jason)
+
+    filtered = ContentFilters::TextMessagePresentationFilters.apply(message.body.body)
+    assert_no_match /data:/, filtered.to_html
+    assert_match /<a>x<\/a>/, filtered.to_html
+  end
+
+  test "message with a safe link and formatting is preserved" do
+    body = '<div><a href="https://example.com">example</a> <strong>bold</strong> <code>code</code><ul><li>one</li><li>two</li></ul></div>'
+    message = Message.create! room: rooms(:pets), body: body, client_message_id: "0015", creator: users(:jason)
+
+    filtered = ContentFilters::TextMessagePresentationFilters.apply(message.body.body)
+    assert_match /<a href="https:\/\/example\.com">example<\/a>/, filtered.to_html
+    assert_match /<strong>bold<\/strong>/, filtered.to_html
+    assert_match /<code>code<\/code>/, filtered.to_html
+    assert_match /<ul>\s*<li>one<\/li>\s*<li>two<\/li>\s*<\/ul>/, filtered.to_html
+  end
+
+  test "SanitizeAttributes uses an isolated sanitizer rather than the shared ActionText instance" do
+    filter = ContentFilters::SanitizeAttributes.new(ActionText::Content.new("<div>x</div>"))
+
+    isolated = filter.send(:sanitizer)
+    assert_not_same ActionText::ContentHelper.sanitizer, isolated,
+      "must not reuse the process-wide ActionText sanitizer, whose permit scrubber is mutated per call"
+    assert_not_same isolated, filter.send(:sanitizer),
+      "each call must build a fresh sanitizer so concurrent requests never share scrubber state"
+    assert_kind_of ActionText::ContentHelper.sanitizer.class, isolated
+  end
+
+  test "SanitizeAttributes neutralizes unsafe input and preserves benign content on its isolated sanitizer" do
+    body = <<~HTML.squish
+      <div><a href="javascript:alert(1)" onclick="x()">link</a>
+      <a href="data:text/html,pwned">data</a>
+      <span class="cf-twitter-avatar" onmouseover="y()">avatar</span>
+      <img src="https://evil.example/x.svg"> Hey #{mention_attachment_for(:david)}</div>
+    HTML
+    message = Message.create! room: rooms(:pets), body: body, client_message_id: "0015", creator: users(:jason)
+
+    filtered = ContentFilters::SanitizeAttributes.apply(message.body.body).to_html
+
+    assert_no_match /javascript:/, filtered
+    assert_no_match /data:text\/html/, filtered
+    assert_no_match /onclick/, filtered
+    assert_no_match /onmouseover/, filtered
+    assert_no_match /evil\.example/, filtered
+    assert_match /<span class="cf-twitter-avatar">avatar<\/span>/, filtered
+    assert_match />link</, filtered
+    assert_match /<action-text-attachment sgid="#{users(:david).attachable_sgid}"/, filtered
+  end
+
   test "message with a mention attachment" do
     message = Message.create! room: rooms(:pets), body: "<div>Hey #{mention_attachment_for(:david)}</div>", creator: users(:jason)
 

test/helpers/messages_helper_test.rb

@@ -0,0 +1,27 @@
+require "test_helper"
+
+class MessagesHelperTest < ActionView::TestCase
+  test "message_presentation neutralizes unsafe URI schemes in links" do
+    message = Message.create! room: rooms(:pets), body: '<div><a href="javascript:alert(1)">x</a></div>', client_message_id: "0015", creator: users(:jason)
+
+    presentation = view.message_presentation(message)
+    assert_no_match /javascript:/, presentation
+    assert_match /<a>x<\/a>/, presentation
+  end
+
+  test "message_presentation strips event handler attributes from allowed tags" do
+    message = Message.create! room: rooms(:pets), body: '<div><a href="/x" onmouseover="alert(1)">x</a></div>', client_message_id: "0015", creator: users(:jason)
+
+    presentation = view.message_presentation(message)
+    assert_no_match /onmouseover/, presentation
+    assert_match /<a href="\/x">x<\/a>/, presentation
+  end
+
+  test "message_presentation preserves safe links and formatting" do
+    message = Message.create! room: rooms(:pets), body: '<div><a href="https://example.com">example</a> <strong>bold</strong></div>', client_message_id: "0015", creator: users(:jason)
+
+    presentation = view.message_presentation(message)
+    assert_match /<a href="https:\/\/example\.com"[^>]*>example<\/a>/, presentation
+    assert_match /<strong>bold<\/strong>/, presentation
+  end
+end

vendor/javascript/trix.esm.min.js

@@ -1,7 +0,0 @@
-/*
-Trix 2.1.19
-Copyright © 2026 37signals, LLC
- */
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