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/changelog.d/fetch-id-containment.security b/changelog.d/fetch-id-containment.security
new file mode 100644
index 000000000..aee5eca95
--- /dev/null
+++ b/changelog.d/fetch-id-containment.security
@@ -0,0 +1 @@
+Only accept fetched objects whose id is the URL they were fetched from; for other ids, refetch once from the canonical id on the same host
diff --git a/changelog.d/get-actor-crash.fix b/changelog.d/get-actor-crash.fix
new file mode 100644
index 000000000..591ac3ba2
--- /dev/null
+++ b/changelog.d/get-actor-crash.fix
@@ -0,0 +1 @@
+Reject fetched objects whose author has no id instead of crashing
diff --git a/changelog.d/refetch-containment.security b/changelog.d/refetch-containment.security
new file mode 100644
index 000000000..bfd118ab1
--- /dev/null
+++ b/changelog.d/refetch-containment.security
@@ -0,0 +1 @@
+Do not let a refetched object (such as a poll) change its id or author
diff --git a/changelog.d/user-fetch-display-url.fix b/changelog.d/user-fetch-display-url.fix
new file mode 100644
index 000000000..fcde4ecf4
--- /dev/null
+++ b/changelog.d/user-fetch-display-url.fix
@@ -0,0 +1 @@
+Update the known user instead of failing when an actor is fetched from a display URL
diff --git a/lib/pleroma/object/containment.ex b/lib/pleroma/object/containment.ex
index c4cc1f014..531100cbb 100644
--- a/lib/pleroma/object/containment.ex
+++ b/lib/pleroma/object/containment.ex
@@ -44,6 +44,9 @@ defmodule Pleroma.Object.Containment do
     id
   end
 
+  # remote data can carry any shape here; callers treat nil as "no actor"
+  def get_actor(_), do: nil
+
   def get_object(%{"object" => id}) when is_binary(id) do
     id
   end
@@ -108,8 +111,46 @@ defmodule Pleroma.Object.Containment do
 
   def contain_origin_from_id(_id, _data), do: :error
 
+  @doc """
+  Checks that a fetched document's id is the URL it was fetched from
+  (after redirects).
+
+  Fragments, a trailing slash, the case of the host and differences
+  that our URL encoding removes before a request are ignored.
+  Documents without an id never match.
+
+  Based on Akkoma's `contain_id_to_fetch/2`.
+  """
+  def contain_id_to_fetch(url, %{"id" => id}) when is_binary(url) and is_binary(id) do
+    if normalize_uri(url) == normalize_uri(id), do: :ok, else: :error
+  end
+
+  def contain_id_to_fetch(_url, _data), do: :error
+
+  defp normalize_uri(uri) do
+    uri = uri |> Pleroma.Utils.URIEncoding.encode_url() |> URI.parse()
+
+    %URI{
+      uri
+      | fragment: nil,
+        host: uri.host && String.downcase(uri.host, :ascii),
+        path: uri.path && String.replace_suffix(uri.path, "/", "")
+    }
+    |> URI.to_string()
+  end
+
   def contain_child(%{"object" => %{"id" => id, "attributedTo" => _} = object}),
     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/object/fetcher.ex b/lib/pleroma/object/fetcher.ex
index 084d44d5e..706ee317c 100644
--- a/lib/pleroma/object/fetcher.ex
+++ b/lib/pleroma/object/fetcher.ex
@@ -11,6 +11,7 @@ defmodule Pleroma.Object.Fetcher do
   alias Pleroma.Web.ActivityPub.InternalFetchActor
   alias Pleroma.Web.ActivityPub.MRF
   alias Pleroma.Web.ActivityPub.ObjectValidator
+  alias Pleroma.Web.ActivityPub.ObjectValidators.CommonFixes
   alias Pleroma.Web.ActivityPub.Pipeline
   alias Pleroma.Web.ActivityPub.Transmogrifier
   alias Pleroma.Web.Federator
@@ -46,9 +47,14 @@ defmodule Pleroma.Object.Fetcher do
     end
   end
 
-  def refetch_object(%Object{data: %{"id" => id}} = object) do
+  # The stored id is canonical, so the refetch must not redirect, and the
+  # remote must not swap in another object or author.
+  # Based on Akkoma 9061d148b and fee57eb37.
+  def refetch_object(%Object{data: %{"id" => id} = data} = object) do
     with {:local, false} <- {:local, Object.local?(object)},
-         {:ok, new_data} <- fetch_and_contain_remote_object_from_id(id),
+         {:ok, new_data} <- fetch_and_contain(id, true),
+         {:id, true} <- {:id, new_data["id"] == id},
+         {:actor, true} <- {:actor, actor_of(new_data) == actor_of(data)},
          {:ok, object} <- reinject_object(object, new_data) do
       {:ok, object}
     else
@@ -57,6 +63,8 @@ defmodule Pleroma.Object.Fetcher do
     end
   end
 
+  defp actor_of(data), do: CommonFixes.fix_actor(data)["actor"]
+
   @typep fetcher_errors ::
            :error | :reject | :allowed_depth | :fetch | :containment | :transmogrifier
 
@@ -140,17 +148,39 @@ defmodule Pleroma.Object.Fetcher do
   def fetch_and_contain_remote_object_from_id(%{"id" => id}),
     do: fetch_and_contain_remote_object_from_id(id)
 
-  def fetch_and_contain_remote_object_from_id(id) when is_binary(id) do
+  def fetch_and_contain_remote_object_from_id(id) when is_binary(id),
+    do: fetch_and_contain(id, false)
+
+  def fetch_and_contain_remote_object_from_id(_id),
+    do: {:error, "id must be a string"}
+
+  # The fetched document's id must be the URL it came from. Display URLs
+  # (e.g. Mastodon's /@user/123) serve the object under another id, so
+  # if the id differs we refetch once from that id, which has to be on the
+  # same host, must not redirect, and must then match exactly.
+  # Based on Akkoma 8684964c5 and 940792f8b.
+  defp fetch_and_contain(id, canonical?) do
     Logger.debug("Fetching object #{id} via AP")
 
     with {:scheme, true} <- {:scheme, String.starts_with?(id, "http")},
          {_, true} <- {:mrf, MRF.id_filter(id)},
          {_, :ok} <- {:local_fetch, Containment.contain_local_fetch(id)},
-         {:ok, body} <- get_object(id),
+         {:ok, final_url, body} <- get_object(id),
+         {_, true} <- {:redirect, not canonical? or same_url?(final_url, id)},
          {:ok, data} <- safe_json_decode(body),
-         :ok <- Containment.contain_origin_from_id(id, data) do
+         :ok <- Containment.contain_origin_from_id(id, data),
+         {_, _, :ok} <- {:id, data, Containment.contain_id_to_fetch(final_url, data)} do
       {:ok, data}
     else
+      {:id, %{"id" => canonical_id}, :error} when is_binary(canonical_id) and not canonical? ->
+        fetch_and_contain(canonical_id, true)
+
+      {:id, _, :error} ->
+        {:error, :id_mismatch}
+
+      {:redirect, false} ->
+        {:error, :canonical_id_redirect}
+
       {:scheme, _} ->
         {:error, "Unsupported URI scheme"}
 
@@ -168,8 +198,7 @@ defmodule Pleroma.Object.Fetcher do
     end
   end
 
-  def fetch_and_contain_remote_object_from_id(_id),
-    do: {:error, "id must be a string"}
+  defp same_url?(url, other), do: Containment.contain_id_to_fetch(url, %{"id" => other}) == :ok
 
   defp check_crossdomain_redirect(final_host, _original_url) when is_nil(final_host) do
     {:cross_domain_redirect, false}
@@ -195,7 +224,7 @@ defmodule Pleroma.Object.Fetcher do
         with {:cross_domain_redirect, false} <- check_crossdomain_redirect(remote_host, id),
              {_, content_type} <- List.keyfind(headers, "content-type", 0),
              {:ok, _media_type} <- verify_content_type(content_type) do
-          {:ok, body}
+          {:ok, if(final_url in [nil, ""], do: id, else: final_url), body}
         else
           {:cross_domain_redirect, true} ->
             {:error, {:cross_domain_redirect, true}}
@@ -209,7 +238,7 @@ defmodule Pleroma.Object.Fetcher do
         case List.keyfind(headers, "content-type", 0) do
           {_, content_type} ->
             case verify_content_type(content_type) do
-              {:ok, _} -> {:ok, body}
+              {:ok, _} -> {:ok, id, body}
               error -> error
             end
 
diff --git a/lib/pleroma/web/activity_pub/activity_pub.ex b/lib/pleroma/web/activity_pub/activity_pub.ex
index 1279e0cb4..b997e7831 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
@@ -2053,11 +2055,12 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
   def enqueue_pin_fetches(_), do: nil
 
   def make_user_from_ap_id(ap_id, additional \\ []) do
-    user = User.get_cached_by_ap_id(ap_id)
-
     with {:ok, data} <- fetch_and_prepare_user_from_ap_id(ap_id, additional) do
       enqueue_pin_fetches(data)
 
+      # the fetched actor's id differs from ap_id when it was fetched from a display URL
+      user = User.get_cached_by_ap_id(data.ap_id)
+
       if user do
         user
         |> User.remote_user_changeset(data)
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..a368ccc16
--- /dev/null
+++ b/lib/pleroma/web/activity_pub/object_validators/user_validator.ex
@@ -0,0 +1,53 @@
+# 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; null and
+  empty strings count as missing.
+
+  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, value) when value in [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/fixtures/tesla_mock/relay@mastdon.example.org.json b/test/fixtures/tesla_mock/relay@mastdon.example.org.json
index c1fab7d3b..21dd405c8 100644
--- a/test/fixtures/tesla_mock/relay@mastdon.example.org.json
+++ b/test/fixtures/tesla_mock/relay@mastdon.example.org.json
@@ -11,7 +11,7 @@
     "toot": "http://joinmastodon.org/ns#",
     "Emoji": "toot:Emoji"
   }],
-  "id": "http://mastodon.example.org/users/admin",
+  "id": "http://mastodon.example.org/users/relay",
   "type": "Application",
   "invisible": true,
   "following": "http://mastodon.example.org/users/admin/following",
@@ -24,8 +24,8 @@
   "url": "http://mastodon.example.org/@admin",
   "manuallyApprovesFollowers": false,
   "publicKey": {
-    "id": "http://mastodon.example.org/users/admin#main-key",
-    "owner": "http://mastodon.example.org/users/admin",
+    "id": "http://mastodon.example.org/users/relay#main-key",
+    "owner": "http://mastodon.example.org/users/relay",
     "publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAtc4Tir+3ADhSNF6VKrtW\nOU32T01w7V0yshmQei38YyiVwVvFu8XOP6ACchkdxbJ+C9mZud8qWaRJKVbFTMUG\nNX4+6Q+FobyuKrwN7CEwhDALZtaN2IPbaPd6uG1B7QhWorrY+yFa8f2TBM3BxnUy\nI4T+bMIZIEYG7KtljCBoQXuTQmGtuffO0UwJksidg2ffCF5Q+K//JfQagJ3UzrR+\nZXbKMJdAw4bCVJYs4Z5EhHYBwQWiXCyMGTd7BGlmMkY6Av7ZqHKC/owp3/0EWDNz\nNqF09Wcpr3y3e8nA10X40MJqp/wR+1xtxp+YGbq/Cj5hZGBG7etFOmIpVBrDOhry\nBwIDAQAB\n-----END PUBLIC KEY-----\n"
   },
   "attachment": [{
diff --git a/test/pleroma/object/containment_test.exs b/test/pleroma/object/containment_test.exs
index 5560dfce5..2fdc83605 100644
--- a/test/pleroma/object/containment_test.exs
+++ b/test/pleroma/object/containment_test.exs
@@ -127,4 +127,56 @@ defmodule Pleroma.Object.ContainmentTest do
       :ok = Containment.contain_child(data)
     end
   end
+
+  describe "get_actor/1" do
+    test "returns nil for actor shapes without an id" do
+      for actor <- [%{"type" => "Person"}, %{"id" => 1}, 1, true] do
+        assert nil == Containment.get_actor(%{"actor" => actor})
+        assert nil == Containment.get_actor(%{"actor" => nil, "attributedTo" => actor})
+      end
+
+      assert nil == Containment.get_actor(%{})
+    end
+
+    test "contain_origin/2 rejects actor shapes without an id" do
+      assert :error ==
+               Containment.contain_origin("https://a.test/o/1", %{
+                 "attributedTo" => %{"type" => "Person"}
+               })
+    end
+  end
+
+  describe "contain_id_to_fetch/2" do
+    test "matches the exact id" do
+      assert :ok =
+               Containment.contain_id_to_fetch("https://a.test/o/1", %{
+                 "id" => "https://a.test/o/1"
+               })
+    end
+
+    test "ignores fragments, a trailing slash and the host case" do
+      assert :ok =
+               Containment.contain_id_to_fetch("https://A.test/o/1/", %{
+                 "id" => "https://a.test/o/1#x"
+               })
+    end
+
+    test "rejects another id on the same host" do
+      assert :error =
+               Containment.contain_id_to_fetch("https://a.test/o/1", %{
+                 "id" => "https://a.test/o/2"
+               })
+    end
+
+    test "rejects a different query" do
+      assert :error =
+               Containment.contain_id_to_fetch("https://a.test/o?page=1", %{
+                 "id" => "https://a.test/o?page=2"
+               })
+    end
+
+    test "rejects documents without an id" do
+      assert :error = Containment.contain_id_to_fetch("https://a.test/o/1", %{})
+    end
+  end
 end
diff --git a/test/pleroma/object/fetcher_test.exs b/test/pleroma/object/fetcher_test.exs
index 9afa34fa2..61a6fb517 100644
--- a/test/pleroma/object/fetcher_test.exs
+++ b/test/pleroma/object/fetcher_test.exs
@@ -48,6 +48,8 @@ defmodule Pleroma.Object.FetcherTest do
         %{method: :get, url: "https://social.sakamoto.gq/notice/9wTkLEnuq47B25EehM"} ->
           %Tesla.Env{
             status: 200,
+            # Pleroma redirects /notice/:id to the object id
+            url: "https://social.sakamoto.gq/objects/f20f2497-66d9-4a52-a2e1-1be2a39c32c1",
             body: File.read!("test/fixtures/fetch_mocks/9wTkLEnuq47B25EehM.json"),
             headers: HttpRequestMock.activitypub_object_headers()
           }
@@ -463,6 +465,78 @@ defmodule Pleroma.Object.FetcherTest do
       %{object1: local_object1, object2: local_object2}
     end
 
+    test "it rejects a refetched object whose actor changed", %{object1: object1} do
+      insert(:user, ap_id: "https://mastodon.social/users/someone_else")
+      {:ok, o} = Object.create(object1)
+
+      mock(fn %{method: :get, url: "https://mastodon.social/1"} ->
+        %Tesla.Env{
+          status: 200,
+          headers: [{"content-type", "application/activity+json"}],
+          body:
+            object1
+            |> Map.merge(%{
+              "actor" => "https://mastodon.social/users/someone_else",
+              "attributedTo" => "https://mastodon.social/users/someone_else",
+              "content" => "hijacked"
+            })
+            |> Jason.encode!()
+        }
+      end)
+
+      assert {:error, _} = Fetcher.refetch_object(o)
+      assert Object.get_by_ap_id("https://mastodon.social/1").data["content"] == "test 1"
+    end
+
+    test "it rejects a refetched object whose author has no id", %{object1: object1} do
+      {:ok, o} = Object.create(object1)
+
+      mock(fn %{method: :get, url: "https://mastodon.social/1"} ->
+        %Tesla.Env{
+          status: 200,
+          headers: [{"content-type", "application/activity+json"}],
+          body:
+            object1
+            |> Map.delete("actor")
+            |> Map.put("attributedTo", %{"type" => "Person"})
+            |> Jason.encode!()
+        }
+      end)
+
+      assert {:error, _} = Fetcher.refetch_object(o)
+    end
+
+    test "it rejects a refetched object whose id changed", %{object1: object1} do
+      {:ok, o} = Object.create(object1)
+
+      mock(fn %{method: :get, url: "https://mastodon.social/1"} ->
+        %Tesla.Env{
+          status: 200,
+          url: "https://mastodon.social/2",
+          headers: [{"content-type", "application/activity+json"}],
+          body: object1 |> Map.put("id", "https://mastodon.social/2") |> Jason.encode!()
+        }
+      end)
+
+      assert {:error, _} = Fetcher.refetch_object(o)
+    end
+
+    test "it rejects a refetched object whose id differs only by a trailing slash", %{
+      object1: object1
+    } do
+      {:ok, o} = Object.create(object1)
+
+      mock(fn %{method: :get, url: "https://mastodon.social/1"} ->
+        %Tesla.Env{
+          status: 200,
+          headers: [{"content-type", "application/activity+json"}],
+          body: object1 |> Map.put("id", "https://mastodon.social/1/") |> Jason.encode!()
+        }
+      end)
+
+      assert {:error, _} = Fetcher.refetch_object(o)
+    end
+
     test "it keeps formerRepresentations if remote does not have this attr", %{object1: object1} do
       full_object1 =
         object1
@@ -642,7 +716,23 @@ defmodule Pleroma.Object.FetcherTest do
               })
           }
 
-        # No redirect - same domain
+        # Same-domain redirect to the object id
+        %{method: :get, url: "https://original.test/@actor/redirect"} ->
+          %Tesla.Env{
+            status: 200,
+            url: "https://original.test/objects/redirect-target",
+            headers: [{"content-type", "application/activity+json"}],
+            body:
+              Jason.encode!(%{
+                "id" => "https://original.test/objects/redirect-target",
+                "type" => "Note",
+                "content" => "This has a same-domain redirect",
+                "actor" => "https://original.test/users/actor",
+                "attributedTo" => "https://original.test/users/actor"
+              })
+          }
+
+        # Same-domain redirect to a document claiming the original URL as id
         %{method: :get, url: "https://original.test/objects/same-domain-redirect"} ->
           %Tesla.Env{
             status: 200,
@@ -695,12 +785,19 @@ defmodule Pleroma.Object.FetcherTest do
     test "it accepts objects with same-domain redirects" do
       assert {:ok, data} =
                Fetcher.fetch_and_contain_remote_object_from_id(
-                 "https://original.test/objects/same-domain-redirect"
+                 "https://original.test/@actor/redirect"
                )
 
       assert data["content"] == "This has a same-domain redirect"
     end
 
+    test "it rejects redirect targets that claim the original URL as id" do
+      assert {:error, _} =
+               Fetcher.fetch_and_contain_remote_object_from_id(
+                 "https://original.test/objects/same-domain-redirect"
+               )
+    end
+
     test "it handles responses without URL field (common in tests)" do
       assert {:ok, data} =
                Fetcher.fetch_and_contain_remote_object_from_id(
@@ -711,6 +808,150 @@ defmodule Pleroma.Object.FetcherTest do
     end
   end
 
+  describe "id containment" do
+    setup do
+      note = fn id, content ->
+        %Tesla.Env{
+          status: 200,
+          headers: [{"content-type", "application/activity+json"}],
+          body:
+            Jason.encode!(%{
+              "id" => id,
+              "type" => "Note",
+              "content" => content,
+              "actor" => "https://host.test/users/actor",
+              "attributedTo" => "https://host.test/users/actor"
+            })
+        }
+      end
+
+      mock(fn
+        # display URL serving the canonical object
+        %{method: :get, url: "https://host.test/@actor/1"} ->
+          note.("https://host.test/objects/1", "display")
+
+        %{method: :get, url: "https://host.test/objects/1"} ->
+          note.("https://host.test/objects/1", "canonical")
+
+        # uploaded file claiming to be another object on the same host
+        %{method: :get, url: "https://host.test/media/evil.json"} ->
+          note.("https://host.test/objects/2", "spoofed")
+
+        %{method: :get, url: "https://host.test/objects/2"} ->
+          note.("https://host.test/objects/2", "real")
+
+        # canonical id does not serve itself
+        %{method: :get, url: "https://host.test/media/loop.json"} ->
+          note.("https://host.test/objects/3", "loop")
+
+        %{method: :get, url: "https://host.test/objects/3"} ->
+          note.("https://host.test/objects/4", "other")
+
+        # canonical id redirects
+        %{method: :get, url: "https://host.test/media/redirect.json"} ->
+          note.("https://host.test/objects/5", "redirect")
+
+        %{method: :get, url: "https://host.test/objects/5"} ->
+          %{note.("https://host.test/objects/5", "moved") | url: "https://host.test/media/x"}
+
+        %{method: :get, url: "https://host.test/objects/slash"} ->
+          note.("https://host.test/objects/slash/", "slash")
+
+        %{method: :get, url: "https://host.test/objects/no-id"} ->
+          note.(nil, "no id")
+      end)
+
+      :ok
+    end
+
+    test "it refetches display URLs from the canonical id" do
+      assert {:ok, %{"id" => "https://host.test/objects/1", "content" => "canonical"}} =
+               Fetcher.fetch_and_contain_remote_object_from_id("https://host.test/@actor/1")
+    end
+
+    test "it does not trust a document claiming another id on the same host" do
+      assert {:ok, %{"id" => "https://host.test/objects/2", "content" => "real"}} =
+               Fetcher.fetch_and_contain_remote_object_from_id(
+                 "https://host.test/media/evil.json"
+               )
+    end
+
+    test "it refetches at most once" do
+      assert {:error, :id_mismatch} =
+               Fetcher.fetch_and_contain_remote_object_from_id(
+                 "https://host.test/media/loop.json"
+               )
+    end
+
+    test "it does not follow redirects when refetching from the canonical id" do
+      assert {:error, _} =
+               Fetcher.fetch_and_contain_remote_object_from_id(
+                 "https://host.test/media/redirect.json"
+               )
+    end
+
+    test "it ignores a trailing slash" do
+      assert {:ok, %{"content" => "slash"}} =
+               Fetcher.fetch_and_contain_remote_object_from_id("https://host.test/objects/slash")
+    end
+
+    test "it accepts ids that only differ by URL encoding" do
+      clear_config(:test_url_encoding, true)
+
+      mock(fn %{method: :get, url: url} ->
+        # the encoder sends uppercase escapes
+        assert url in [
+                 "https://host.test/users/%E3%81%82",
+                 "https://host.test/notes/%E3%81%82"
+               ]
+
+        id =
+          if url =~ "users",
+            do: "https://host.test/users/あ",
+            else: "https://host.test/notes/%e3%81%82"
+
+        %Tesla.Env{
+          status: 200,
+          url: url,
+          headers: [{"content-type", "application/activity+json"}],
+          body: Jason.encode!(%{"id" => id, "type" => "Note"})
+        }
+      end)
+
+      assert {:ok, _} =
+               Fetcher.fetch_and_contain_remote_object_from_id("https://host.test/users/あ")
+
+      assert {:ok, _} =
+               Fetcher.fetch_and_contain_remote_object_from_id(
+                 "https://host.test/notes/%e3%81%82"
+               )
+    end
+
+    test "it rejects objects whose author has no id" do
+      mock(fn %{method: :get, url: "https://host.test/objects/anon" = url} ->
+        %Tesla.Env{
+          status: 200,
+          headers: [{"content-type", "application/activity+json"}],
+          body:
+            Jason.encode!(%{
+              "id" => url,
+              "type" => "Note",
+              "content" => "anon",
+              "attributedTo" => %{"type" => "Person", "name" => "nobody"}
+            })
+        }
+      end)
+
+      assert {:containment, :error} =
+               Fetcher.fetch_object_from_id("https://host.test/objects/anon")
+    end
+
+    test "it rejects documents without an id" do
+      assert {:error, _} =
+               Fetcher.fetch_and_contain_remote_object_from_id("https://host.test/objects/no-id")
+    end
+  end
+
   describe "fetch with history" do
     setup do
       object2 = %{
diff --git a/test/pleroma/web/activity_pub/activity_pub_test.exs b/test/pleroma/web/activity_pub/activity_pub_test.exs
index ed6db558b..c118cd8f1 100644
--- a/test/pleroma/web/activity_pub/activity_pub_test.exs
+++ b/test/pleroma/web/activity_pub/activity_pub_test.exs
@@ -244,6 +244,58 @@ 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 "updates the known user when fetched from a display URL" do
+      ap_id = "https://example.com/users/lain"
+      display_url = "https://example.com/@lain"
+      user = insert(:user, local: false, ap_id: ap_id, nickname: "lain@example.com")
+
+      user_data =
+        "test/fixtures/users_mock/user.json"
+        |> File.read!()
+        |> String.replace("{{nickname}}", "lain")
+        |> Jason.decode!()
+        |> Map.merge(%{"featured" => nil, "name" => "new name"})
+        |> Jason.encode!()
+
+      Tesla.Mock.mock(fn %{method: :get, url: url} when url in [ap_id, display_url] ->
+        %Tesla.Env{
+          status: 200,
+          body: user_data,
+          headers: [{"content-type", "application/activity+json"}]
+        }
+      end)
+
+      assert {:ok, %User{id: id, name: "new name"}} =
+               ActivityPub.make_user_from_ap_id(display_url)
+
+      assert id == user.id
+    end
+
     test "fetches user featured collection" do
       ap_id = "https://example.com/users/lain"
 
@@ -516,7 +568,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubTest do
     end
 
     test "fetches user birthday information from misskey" do
-      user_id = "https://misskey.io/@mkljczk"
+      user_id = "https://misskey.io/users/8dhi2ne167"
 
       Tesla.Mock.mock(fn
         %{
@@ -2033,7 +2085,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubTest do
             json(
               %{
                 "@context" => "https://www.w3.org/ns/activitystreams",
-                "id" => "https://remote.org/users/masto_hidden_counters/followers"
+                "id" => "https://remote.org/users/masto_hidden_counters/following"
               },
               headers: HttpRequestMock.activitypub_object_headers()
             )
@@ -2045,7 +2097,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubTest do
             json(
               %{
                 "@context" => "https://www.w3.org/ns/activitystreams",
-                "id" => "https://remote.org/users/masto_hidden_counters/following"
+                "id" => "https://remote.org/users/masto_hidden_counters/followers"
               },
               headers: HttpRequestMock.activitypub_object_headers()
             )
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..fbc89462e
--- /dev/null
+++ b/test/pleroma/web/activity_pub/object_validators/user_validator_test.exs
@@ -0,0 +1,75 @@
+# 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
+
+  test "treats empty and null fields as missing" do
+    data =
+      actor()
+      |> Map.merge(%{"inbox" => "", "followers" => nil, "following" => "", "featured" => ""})
+      |> put_in(["endpoints", "sharedInbox"], "")
+      |> put_in(["publicKey", "id"], "")
+
+    assert {:ok, _, _} = UserValidator.validate(data, [])
+  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!()
 
diff --git a/test/support/http_request_mock.ex b/test/support/http_request_mock.ex
index 82affe7ac..94be59536 100644
--- a/test/support/http_request_mock.ex
+++ b/test/support/http_request_mock.ex
@@ -577,6 +577,16 @@ defmodule HttpRequestMock do
      }}
   end
 
+  # the same status via its canonical id
+  def get("http://mastodon.example.org/users/admin/statuses/99541947525187367", _, _, _) do
+    {:ok,
+     %Tesla.Env{
+       status: 200,
+       body: File.read!("test/fixtures/mastodon-note-object.json"),
+       headers: activitypub_object_headers()
+     }}
+  end
+
   def get("http://mastodon.example.org/@admin/99541947525187368", _, _, _) do
     {:ok,
      %Tesla.Env{