Chat message creation: Check actor.

lib/pleroma/web/activity_pub/object_validators/create_chat_message_validator.ex

@@ -45,6 +45,7 @@ def validate_data(cng, meta \\ []) do
     |> validate_inclusion(:type, ["Create"])
     |> validate_actor_presence()
     |> validate_recipients_match(meta)
+    |> validate_actors_match(meta)
     |> validate_object_nonexistence()
   end
 
@@ -59,6 +60,19 @@ def validate_object_nonexistence(cng) do
     end)
   end
 
+  def validate_actors_match(cng, meta) do
+    object_actor = meta[:object_data]["actor"]
+
+    cng
+    |> validate_change(:actor, fn :actor, actor ->
+      if actor == object_actor do
+        []
+      else
+        [{:actor, "Actor doesn't match with object actor"}]
+      end
+    end)
+  end
+
   def validate_recipients_match(cng, meta) do
     object_recipients = meta[:object_data]["to"] || []
 

test/web/activity_pub/object_validator_test.exs

@@ -23,6 +23,19 @@ test "it is invalid if the object already exists" do
 
       assert {:object, {"The object to create already exists", []}} in cng.errors
     end
+
+    test "it is invalid if the object data has a different `to` or `actor` field" do
+      user = insert(:user)
+      recipient = insert(:user)
+      {:ok, object_data, _} = Builder.chat_message(recipient, user.ap_id, "Hey")
+
+      {:ok, create_data, _} = Builder.create(user, object_data, [recipient.ap_id])
+
+      {:error, cng} = ObjectValidator.validate(create_data, [])
+
+      assert {:to, {"Recipients don't match with object recipients", []}} in cng.errors
+      assert {:actor, {"Actor doesn't match with object actor", []}} in cng.errors
+    end
   end
 
   describe "chat messages" do