Harden message content filtering + bump thruster to 0.1.23 (#237)

* Bump thruster 0.1.15 → 0.1.23

* Scrub disallowed attributes on allowed tags in message rendering

SanitizeTags removes disallowed tags from message presentation, but
attributes on the tags it allows passed through untouched. Extend the
content-filter chain with a SanitizeAttributes filter that runs Rails'
safe-list sanitizer over the remaining markup, stripping event-handler
attributes and unsafe URI schemes as defense-in-depth alongside the
existing Content-Security-Policy.

Running it after SanitizeTags with the same allowed-tags list makes the
sanitizer's tag pass a no-op, preserving SanitizeTags' remove-not-unwrap
semantics while scrubbing attributes. The attribute allowlist is the
standard ActionText set (which keeps attachments intact) plus class,
which presentation styling relies on.

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>