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diff --git a/lib/pleroma/web/activity_pub/object_validators/undo_validator.ex b/lib/pleroma/web/activity_pub/object_validators/undo_validator.ex
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+++ b/lib/pleroma/web/activity_pub/object_validators/undo_validator.ex
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+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2020 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.ObjectValidators.UndoValidator do
+  use Ecto.Schema
+
+  alias Pleroma.Activity
+  alias Pleroma.EctoType.ActivityPub.ObjectValidators
+
+  import Ecto.Changeset
+  import Pleroma.Web.ActivityPub.ObjectValidators.CommonValidations
+
+  @primary_key false
+
+  embedded_schema do
+    field(:id, ObjectValidators.ObjectID, primary_key: true)
+    field(:type, :string)
+    field(:object, ObjectValidators.ObjectID)
+    field(:actor, ObjectValidators.ObjectID)
+    field(:to, ObjectValidators.Recipients, default: [])
+    field(:cc, ObjectValidators.Recipients, default: [])
+  end
+
+  def cast_and_validate(data) do
+    data
+    |> cast_data()
+    |> validate_data()
+  end
+
+  def cast_data(data) do
+    %__MODULE__{}
+    |> changeset(data)
+  end
+
+  def changeset(struct, data) do
+    struct
+    |> cast(data, __schema__(:fields))
+  end
+
+  def validate_data(data_cng) do
+    data_cng
+    |> validate_inclusion(:type, ["Undo"])
+    |> validate_required([:id, :type, :object, :actor, :to, :cc])
+    |> validate_actor_presence()
+    |> validate_object_presence()
+    |> validate_undo_rights()
+  end
+
+  def validate_undo_rights(cng) do
+    actor = get_field(cng, :actor)
+    object = get_field(cng, :object)
+
+    with %Activity{data: %{"actor" => object_actor}} <- Activity.get_by_ap_id(object),
+         true <- object_actor != actor do
+      cng
+      |> add_error(:actor, "not the same as object actor")
+    else
+      _ -> cng
+    end
+  end
+end