diff --git a/changelog.d/actor-containment.security b/changelog.d/actor-containment.security
new file mode 100644
index 000000000..58dbca4bc
--- /dev/null
+++ b/changelog.d/actor-containment.security
@@ -0,0 +1 @@
+Reject remote actors whose inbox, shared inbox, followers, following or featured collection, or key id is on another host
diff --git a/lib/pleroma/object/containment.ex b/lib/pleroma/object/containment.ex
index c4cc1f014..b50d560d1 100644
--- a/lib/pleroma/object/containment.ex
+++ b/lib/pleroma/object/containment.ex
@@ -112,4 +112,14 @@ defmodule Pleroma.Object.Containment do
     do: contain_origin(id, object)
 
   def contain_child(_), do: :ok
+
+  @doc "Checks that two URIs are on the same host. URIs without a host never match."
+  def same_origin(id, other_id) when is_binary(id) and is_binary(other_id) do
+    case {URI.parse(id), URI.parse(other_id)} do
+      {%URI{host: host}, %URI{host: host}} when is_binary(host) and host != "" -> :ok
+      _ -> :error
+    end
+  end
+
+  def same_origin(_id, _other_id), do: :error
 end
diff --git a/lib/pleroma/web/activity_pub/activity_pub.ex b/lib/pleroma/web/activity_pub/activity_pub.ex
index 1279e0cb4..18cb2cadc 100644
--- a/lib/pleroma/web/activity_pub/activity_pub.ex
+++ b/lib/pleroma/web/activity_pub/activity_pub.ex
@@ -21,6 +21,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
   alias Pleroma.Upload
   alias Pleroma.User
   alias Pleroma.Web.ActivityPub.MRF
+  alias Pleroma.Web.ActivityPub.ObjectValidators.UserValidator
   alias Pleroma.Web.ActivityPub.Transmogrifier
   alias Pleroma.Web.Streamer
   alias Pleroma.Web.WebFinger
@@ -1924,6 +1925,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
 
   def user_data_from_user_object(data, additional \\ []) do
     with {:ok, data} <- MRF.filter(data),
+         {:ok, data, _} <- UserValidator.validate(data, []),
          {:ok, data} <- object_to_user_data(data, additional) do
       {:ok, data}
     else
diff --git a/lib/pleroma/web/activity_pub/object_validator.ex b/lib/pleroma/web/activity_pub/object_validator.ex
index 0254d6dc1..468298a4d 100644
--- a/lib/pleroma/web/activity_pub/object_validator.ex
+++ b/lib/pleroma/web/activity_pub/object_validator.ex
@@ -11,7 +11,7 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidator do
 
   @behaviour Pleroma.Web.ActivityPub.ObjectValidator.Validating
 
-  import Pleroma.Constants, only: [activity_types: 0, object_types: 0]
+  import Pleroma.Constants, only: [activity_types: 0, actor_types: 0, object_types: 0]
 
   alias Pleroma.Activity
   alias Pleroma.EctoType.ActivityPub.ObjectValidators
@@ -37,6 +37,7 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidator do
   alias Pleroma.Web.ActivityPub.ObjectValidators.QuestionValidator
   alias Pleroma.Web.ActivityPub.ObjectValidators.UndoValidator
   alias Pleroma.Web.ActivityPub.ObjectValidators.UpdateValidator
+  alias Pleroma.Web.ActivityPub.ObjectValidators.UserValidator
 
   @impl true
   def validate(object, meta)
@@ -226,7 +227,8 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidator do
   end
 
   def validate(%{"type" => type} = object, meta) when type == "Update" do
-    with {:ok, object} <-
+    with :ok <- validate_updated_actor(object, meta),
+         {:ok, object} <-
            object
            |> UpdateValidator.cast_and_validate(meta)
            |> Ecto.Changeset.apply_action(:insert) do
@@ -247,6 +249,19 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidator do
 
   def validate(o, m), do: {:error, {:validator_not_set, {o, m}}}
 
+  defp validate_updated_actor(%{"object" => %{"type" => type} = actor}, meta)
+       when type in actor_types() do
+    with {_, false} <- {:local, Access.get(meta, :local, false)},
+         {:ok, _, _} <- UserValidator.validate(actor, meta) do
+      :ok
+    else
+      {:local, true} -> :ok
+      error -> error
+    end
+  end
+
+  defp validate_updated_actor(_, _), do: :ok
+
   def cast_and_apply_and_stringify_with_history(object) do
     do_separate_with_history(object, fn object ->
       with {:ok, object_data} <- cast_and_apply(object),
diff --git a/lib/pleroma/web/activity_pub/object_validators/user_validator.ex b/lib/pleroma/web/activity_pub/object_validators/user_validator.ex
new file mode 100644
index 000000000..183be4940
--- /dev/null
+++ b/lib/pleroma/web/activity_pub/object_validators/user_validator.ex
@@ -0,0 +1,52 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.ObjectValidators.UserValidator do
+  @moduledoc """
+  Checks that a remote actor document only points at its own host.
+
+  We deliver to the inbox and shared inbox, use the collections for
+  addressing and visibility checks, and map signature key ids back to
+  the actor. If an actor could point any of these at another host, it
+  could redirect our signed deliveries or claim another actor's
+  collections. Optional fields are only checked when present.
+
+  Based on Akkoma's UserValidator.
+  """
+
+  @behaviour Pleroma.Web.ActivityPub.ObjectValidator.Validating
+
+  alias Pleroma.Object.Containment
+
+  @collections ["inbox", "followers", "following", "featured"]
+
+  @impl true
+  def validate(%{"id" => id} = data, meta) when is_binary(id) do
+    fields =
+      Enum.map(@collections, &{&1, data[&1]}) ++
+        [
+          {"sharedInbox", get_in_map(data, ["endpoints", "sharedInbox"])},
+          {"publicKey", get_in_map(data, ["publicKey", "id"])}
+        ]
+
+    case Enum.find(fields, fn {_, value} -> not same_origin?(id, value) end) do
+      nil -> {:ok, data, meta}
+      {field, _} -> {:error, "#{field} is not on the same host as the actor"}
+    end
+  end
+
+  def validate(_data, _meta), do: {:error, "Actor has no id"}
+
+  defp same_origin?(_id, nil), do: true
+  defp same_origin?(id, %{"id" => value}), do: same_origin?(id, value)
+  defp same_origin?(id, value), do: Containment.same_origin(id, value) == :ok
+
+  defp get_in_map(data, [key | rest]) do
+    case data do
+      %{^key => value} when rest == [] -> value
+      %{^key => value} -> get_in_map(value, rest)
+      _ -> nil
+    end
+  end
+end
diff --git a/test/fixtures/bridgy/actor.json b/test/fixtures/bridgy/actor.json
index 5b2d8982b..b4e859a82 100644
--- a/test/fixtures/bridgy/actor.json
+++ b/test/fixtures/bridgy/actor.json
@@ -70,7 +70,7 @@
   "preferredUsername": "jk.nipponalba.scot",
   "summary": "",
   "publicKey": {
-    "id": "jk.nipponalba.scot",
+    "id": "https://fed.brid.gy/jk.nipponalba.scot#key",
     "publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDdarxwzxnNbJ2hneWOYHkYJowk\npyigQtxlUd0VjgSQHwxU9kWqfbrHBVADyTtcqi/4dAzQd3UnCI1TPNnn4LPZY9PW\noiWd3Zl1/EfLFxO7LU9GS7fcSLQkyj5JNhSlN3I8QPudZbybrgRDVZYooDe1D+52\n5KLGqC2ajrIVOiDRTQIDAQAB\n-----END PUBLIC KEY-----"
   },
   "inbox": "https://fed.brid.gy/jk.nipponalba.scot/inbox",
diff --git a/test/pleroma/web/activity_pub/activity_pub_test.exs b/test/pleroma/web/activity_pub/activity_pub_test.exs
index ed6db558b..125a2b4f9 100644
--- a/test/pleroma/web/activity_pub/activity_pub_test.exs
+++ b/test/pleroma/web/activity_pub/activity_pub_test.exs
@@ -244,6 +244,31 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubTest do
              }
     end
 
+    test "rejects actors whose inbox is on another host" do
+      ap_id = "https://example.com/users/evil"
+
+      user_data =
+        "test/fixtures/users_mock/user.json"
+        |> File.read!()
+        |> String.replace("{{nickname}}", "evil")
+        |> Jason.decode!()
+        |> Map.put("inbox", "https://victim.example/inbox")
+        |> Map.put("featured", nil)
+        |> Jason.encode!()
+
+      Tesla.Mock.mock(fn
+        %{method: :get, url: ^ap_id} ->
+          %Tesla.Env{
+            status: 200,
+            body: user_data,
+            headers: [{"content-type", "application/activity+json"}]
+          }
+      end)
+
+      assert {:error, _} = ActivityPub.make_user_from_ap_id(ap_id)
+      refute User.get_by_ap_id(ap_id)
+    end
+
     test "fetches user featured collection" do
       ap_id = "https://example.com/users/lain"
 
diff --git a/test/pleroma/web/activity_pub/object_validators/user_validator_test.exs b/test/pleroma/web/activity_pub/object_validators/user_validator_test.exs
new file mode 100644
index 000000000..91b33b9e2
--- /dev/null
+++ b/test/pleroma/web/activity_pub/object_validators/user_validator_test.exs
@@ -0,0 +1,65 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.ObjectValidators.UserValidatorTest do
+  use ExUnit.Case, async: true
+
+  alias Pleroma.Web.ActivityPub.ObjectValidators.UserValidator
+
+  defp actor do
+    "test/fixtures/users_mock/user.json"
+    |> File.read!()
+    |> String.replace("{{nickname}}", "lain")
+    |> Jason.decode!()
+  end
+
+  test "accepts an actor whose endpoints and collections are on its own host" do
+    data = actor()
+    assert {:ok, ^data, []} = UserValidator.validate(data, [])
+  end
+
+  test "accepts an actor without optional fields" do
+    data = Map.drop(actor(), ["endpoints", "featured", "followers", "following", "publicKey"])
+    assert {:ok, _, _} = UserValidator.validate(data, [])
+  end
+
+  test "accepts all actor fixtures" do
+    for path <- Path.wildcard("test/fixtures/**/*.json"),
+        {:ok, %{"inbox" => _, "id" => id} = data} when is_binary(id) <-
+          [Jason.decode(File.read!(path))] do
+      assert {{:ok, _, _}, ^path} = {UserValidator.validate(data, []), path}
+    end
+  end
+
+  for field <- ["inbox", "followers", "following", "featured"] do
+    test "rejects an actor whose #{field} is on another host" do
+      data = Map.put(actor(), unquote(field), "https://victim.example/users/lain/x")
+      assert {:error, _} = UserValidator.validate(data, [])
+    end
+  end
+
+  test "rejects an actor whose sharedInbox is on another host" do
+    data = put_in(actor(), ["endpoints", "sharedInbox"], "https://victim.example/inbox")
+    assert {:error, _} = UserValidator.validate(data, [])
+  end
+
+  test "rejects an actor whose collection is an object on another host" do
+    data = Map.put(actor(), "followers", %{"id" => "https://victim.example/users/v/followers"})
+    assert {:error, _} = UserValidator.validate(data, [])
+  end
+
+  test "rejects an actor whose key id is on another host" do
+    data = put_in(actor(), ["publicKey", "id"], "https://victim.example/users/lain#main-key")
+    assert {:error, _} = UserValidator.validate(data, [])
+  end
+
+  test "rejects an actor whose key id has no host" do
+    data = put_in(actor(), ["publicKey", "id"], "lain")
+    assert {:error, _} = UserValidator.validate(data, [])
+  end
+
+  test "rejects an actor without an id" do
+    assert {:error, _} = UserValidator.validate(Map.delete(actor(), "id"), [])
+  end
+end
diff --git a/test/pleroma/web/activity_pub/transmogrifier/user_update_handling_test.exs b/test/pleroma/web/activity_pub/transmogrifier/user_update_handling_test.exs
index cf995e143..b39bc8780 100644
--- a/test/pleroma/web/activity_pub/transmogrifier/user_update_handling_test.exs
+++ b/test/pleroma/web/activity_pub/transmogrifier/user_update_handling_test.exs
@@ -11,8 +11,11 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier.UserUpdateHandlingTest do
 
   import Pleroma.Factory
 
+  # matches the host of the actor in mastodon-update.json
+  @gargron "http://mastodon.example.org/users/gargron"
+
   test "it works for incoming update activities" do
-    user = insert(:user, local: false)
+    user = insert(:user, local: false, ap_id: @gargron)
 
     update_data = File.read!("test/fixtures/mastodon-update.json") |> Jason.decode!()
 
@@ -50,8 +53,28 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier.UserUpdateHandlingTest do
     assert user.bio == "<p>Some bio</p>"
   end
 
+  test "it rejects updates that move the shared inbox to another host" do
+    user = insert(:user, local: false, ap_id: @gargron)
+
+    update_data = File.read!("test/fixtures/mastodon-update.json") |> Jason.decode!()
+
+    object =
+      update_data["object"]
+      |> Map.put("actor", user.ap_id)
+      |> Map.put("id", user.ap_id)
+      |> put_in(["endpoints", "sharedInbox"], "https://victim.example/inbox")
+
+    update_data =
+      update_data
+      |> Map.put("actor", user.ap_id)
+      |> Map.put("object", object)
+
+    assert {:error, _} = Transmogrifier.handle_incoming(update_data)
+    assert User.get_cached_by_ap_id(user.ap_id).shared_inbox == user.shared_inbox
+  end
+
   test "it selects an actor type after an extension type" do
-    user = insert(:user, local: false)
+    user = insert(:user, local: false, ap_id: @gargron)
     update_data = File.read!("test/fixtures/mastodon-update.json") |> Jason.decode!()
 
     object =
@@ -70,7 +93,7 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier.UserUpdateHandlingTest do
   end
 
   test "it works with alsoKnownAs" do
-    %{ap_id: actor} = insert(:user, local: false)
+    %{ap_id: actor} = insert(:user, local: false, ap_id: @gargron)
 
     assert User.get_cached_by_ap_id(actor).also_known_as == []
 
@@ -97,7 +120,7 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier.UserUpdateHandlingTest do
   end
 
   test "it works with custom profile fields" do
-    user = insert(:user, local: false)
+    user = insert(:user, local: false, ap_id: @gargron)
 
     assert user.fields == []
 
@@ -155,7 +178,7 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier.UserUpdateHandlingTest do
   end
 
   test "it works for incoming update activities which lock the account" do
-    user = insert(:user, local: false)
+    user = insert(:user, local: false, ap_id: @gargron)
 
     update_data = File.read!("test/fixtures/mastodon-update.json") |> Jason.decode!()