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>

3 files changed, 32 insertions, 0 deletions

diff --git a/app/lib/activitypub/activity.rb b/app/lib/activitypub/activity.rb
index 0ce279d28..ab946470b 100644
--- a/app/lib/activitypub/activity.rb
+++ b/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
diff --git a/app/lib/activitypub/activity/create.rb b/app/lib/activitypub/activity/create.rb
index e81452e3c..08dd98e94 100644
--- a/app/lib/activitypub/activity/create.rb
+++ b/app/lib/activitypub/activity/create.rb
@@ -2,6 +2,8 @@
 
 class ActivityPub::Activity::Create < ActivityPub::Activity
   def perform
+    dereference_object!
+
     case @object['type']
     when 'EncryptedMessage'
       create_encrypted_message
diff --git a/app/lib/activitypub/activity/update.rb b/app/lib/activitypub/activity/update.rb
index 70035325b..018e2df54 100644
--- a/app/lib/activitypub/activity/update.rb
+++ b/app/lib/activitypub/activity/update.rb
@@ -4,6 +4,8 @@ class ActivityPub::Activity::Update < ActivityPub::Activity
   SUPPORTED_TYPES = %w(Application Group Organization Person Service).freeze
 
   def perform
+    dereference_object!
+
     if equals_or_includes_any?(@object['type'], SUPPORTED_TYPES)
       update_account
     elsif equals_or_includes_any?(@object['type'], %w(Question))