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diff --git a/lib/pleroma/object/containment.ex b/lib/pleroma/object/containment.ex
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+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Object.Containment do
+  @moduledoc """
+  This module contains some useful functions for containing objects to specific
+  origins and determining those origins.  They previously lived in the
+  ActivityPub `Transmogrifier` module.
+
+  Object containment is an important step in validating remote objects to prevent
+  spoofing, therefore removal of object containment functions is NOT recommended.
+  """
+  def get_actor(%{"actor" => actor}) when is_binary(actor) do
+    actor
+  end
+
+  def get_actor(%{"actor" => actor}) when is_list(actor) do
+    if is_binary(Enum.at(actor, 0)) do
+      Enum.at(actor, 0)
+    else
+      Enum.find(actor, fn %{"type" => type} -> type in ["Person", "Service", "Application"] end)
+      |> Map.get("id")
+    end
+  end
+
+  def get_actor(%{"actor" => %{"id" => id}}) when is_bitstring(id) do
+    id
+  end
+
+  def get_actor(%{"actor" => nil, "attributedTo" => actor}) when not is_nil(actor) do
+    get_actor(%{"actor" => actor})
+  end
+
+  def get_object(%{"object" => id}) when is_binary(id) do
+    id
+  end
+
+  def get_object(%{"object" => %{"id" => id}}) when is_binary(id) do
+    id
+  end
+
+  def get_object(_) do
+    nil
+  end
+
+  defp compare_uris(%URI{host: host} = _id_uri, %URI{host: host} = _other_uri), do: :ok
+  defp compare_uris(_id_uri, _other_uri), do: :error
+
+  @doc """
+  Checks that an imported AP object's actor matches the host it came from.
+  """
+  def contain_origin(_id, %{"actor" => nil}), do: :error
+
+  def contain_origin(id, %{"actor" => _actor} = params) do
+    id_uri = URI.parse(id)
+    actor_uri = URI.parse(get_actor(params))
+
+    compare_uris(actor_uri, id_uri)
+  end
+
+  def contain_origin(id, %{"attributedTo" => actor} = params),
+    do: contain_origin(id, Map.put(params, "actor", actor))
+
+  def contain_origin(_id, _data), do: :error
+
+  def contain_origin_from_id(id, %{"id" => other_id} = _params) when is_binary(other_id) do
+    id_uri = URI.parse(id)
+    other_uri = URI.parse(other_id)
+
+    compare_uris(id_uri, other_uri)
+  end
+
+  # Mastodon pin activities don't have an id, so we check the object field, which will be pinned.
+  def contain_origin_from_id(id, %{"object" => object}) when is_binary(object) do
+    id_uri = URI.parse(id)
+    object_uri = URI.parse(object)
+
+    compare_uris(id_uri, object_uri)
+  end
+
+  def contain_origin_from_id(_id, _data), do: :error
+
+  def contain_child(%{"object" => %{"id" => id, "attributedTo" => _} = object}),
+    do: contain_origin(id, object)
+
+  def contain_child(_), do: :ok
+end