Merge pull request from GHSA-ccm4-vgcc-73hp

* Tighten allowed HTML in oEmbed-based preview cards

* Sanitize preview cards at render time

* Add `sandbox` attribute to preview card iframes

app/serializers/rest/preview_card_serializer.rb

@@ -11,4 +11,8 @@ class REST::PreviewCardSerializer < ActiveModel::Serializer
   def image
     object.image? ? full_asset_url(object.image.url(:original)) : nil
   end
+
+  def html
+    Sanitize.fragment(object.html, Sanitize::Config::MASTODON_OEMBED)
+  end
 end

lib/sanitize_ext/sanitize_config.rb

@@ -94,26 +94,26 @@ class Sanitize
       ]
     )
 
-    MASTODON_OEMBED ||= freeze_config merge(
-      RELAXED,
-      elements: RELAXED[:elements] + %w(audio embed iframe source video),
+    MASTODON_OEMBED ||= freeze_config(
+      elements: %w(audio embed iframe source video),
 
-      attributes: merge(
-        RELAXED[:attributes],
+      attributes: {
         'audio'  => %w(controls),
         'embed'  => %w(height src type width),
         'iframe' => %w(allowfullscreen frameborder height scrolling src width),
         'source' => %w(src type),
         'video'  => %w(controls height loop width),
-        'div'    => [:data]
-      ),
+      },
 
-      protocols: merge(
-        RELAXED[:protocols],
+      protocols: {
         'embed'  => { 'src' => HTTP_PROTOCOLS },
         'iframe' => { 'src' => HTTP_PROTOCOLS },
-        'source' => { 'src' => HTTP_PROTOCOLS }
-      )
+        'source' => { 'src' => HTTP_PROTOCOLS },
+      },
+
+      add_attributes: {
+        'iframe' => { 'sandbox' => 'allow-scripts allow-same-origin allow-popups allow-popups-to-escape-sandbox allow-forms' },
+      }
     )
   end
 end