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