Dereference object URIs in Create and Update messages (#14359)

* Dereference object URIs in Create and Update messages

Fixes #14353

Signed-off-by: Thibaut Girka <thib@sitedethib.com>

* Refactor, and perform origin check *before* attempting to fetch object

Co-authored-by: Fire Demon <firedemon@creature.cafe>

app/lib/activitypub/activity.rb

@@ -157,6 +157,34 @@ class ActivityPub::Activity
     fetch_remote_original_status
   end
 
+  def dereference_object!
+    return unless @object.is_a?(String)
+    return if invalid_origin?(@object)
+
+    object = fetch_resource(@object, true, signed_fetch_account)
+    return unless object.present? && object.is_a?(Hash) && supported_context?(object)
+
+    @object = object
+  end
+
+  def signed_fetch_account
+    first_mentioned_local_account || first_local_follower
+  end
+
+  def first_mentioned_local_account
+    audience = (as_array(@json['to']) + as_array(@json['cc'])).uniq
+    local_usernames = audience.select { |uri| ActivityPub::TagManager.instance.local_uri?(uri) }
+                              .map { |uri| ActivityPub::TagManager.instance.uri_to_local_id(uri, :username) }
+
+    return if local_usernames.empty?
+
+    Account.local.where(username: local_usernames).first
+  end
+
+  def first_local_follower
+    @account.followers.local.first
+  end
+
   def follow_request_from_object
     @follow_request ||= FollowRequest.find_by(target_account: @account, uri: object_uri) unless object_uri.nil?
   end