Convert to an Plug called InboxGuard

lib/pleroma/constants.ex

@@ -85,6 +85,18 @@ defmodule Pleroma.Constants do
     ]
   )
 
+  const(allowed_activity_types_from_strangers,
+    do: [
+      "Block",
+      "Create",
+      "Flag",
+      "Follow",
+      "Like",
+      "Move",
+      "React"
+    ]
+  )
+
   # basic regex, just there to weed out potential mistakes
   # https://datatracker.ietf.org/doc/html/rfc2045#section-5.1
   const(mime_regex,

lib/pleroma/web/activity_pub/activity_pub_controller.ex

@@ -294,11 +294,6 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubController do
   end
 
   def inbox(%{assigns: %{valid_signature: false}} = conn, params) do
-    case unknown_delete?(params) do
-      true ->
-        :ok
-
-      false ->
     Federator.incoming_ap_doc(%{
       method: conn.method,
       req_headers: conn.req_headers,
@@ -306,7 +301,6 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubController do
       params: params,
       query_string: conn.query_string
     })
-    end
 
     json(conn, "ok")
   end
@@ -564,17 +558,4 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubController do
       |> json(UserView.render("featured.json", %{user: user}))
     end
   end
-
-  defp unknown_delete?(
-         %{
-           "type" => "Delete"
-         } = data
-       ) do
-    case data |> Pleroma.Object.Containment.get_actor() |> User.get_cached_by_ap_id() do
-      %User{} -> false
-      _ -> true
-    end
-  end
-
-  defp unknown_delete?(_), do: false
 end

lib/pleroma/web/plugs/inbox_guard_plug.ex

@@ -0,0 +1,66 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.Plugs.InboxGuardPlug do
+  import Plug.Conn
+  import Pleroma.Constants, only: [allowed_activity_types_from_strangers: 0]
+
+  alias Pleroma.Config
+  alias Pleroma.User
+
+  def init(options) do
+    options
+  end
+
+  def call(%{assigns: %{valid_signature: true}} = conn, _opts) do
+    conn
+  end
+
+  def call(conn, _opts) do
+    with {_, true} <- {:federating, Config.get!([:instance, :federating])},
+         true <- known_actor?(conn) do
+      conn
+    else
+      {:federating, false} ->
+        conn
+        |> json(403, "Not federating")
+
+      _ ->
+        conn
+        |> filter_from_strangers()
+    end
+  end
+
+  # If signature failed but we know this actor we should
+  # accept it as we may only need to refetch their public key
+  # during processing
+  defp known_actor?(%{body_params: data}) do
+    case Pleroma.Object.Containment.get_actor(data) |> User.get_cached_by_ap_id() do
+      %User{} -> true
+      _ -> false
+    end
+  end
+
+  # Only permit a subset of activity types from strangers
+  # or else it will add actors you've never interacted with
+  # to the database
+  defp filter_from_strangers(%{body_params: %{"type" => type}} = conn) do
+    with true <- type in allowed_activity_types_from_strangers() do
+      conn
+    else
+      _ ->
+        conn
+        |> json(400, "Invalid activity type for an unknown actor")
+    end
+  end
+
+  defp json(conn, status, resp) do
+    json_resp = Jason.encode!(resp)
+
+    conn
+    |> put_resp_content_type("application/json")
+    |> resp(status, json_resp)
+    |> halt()
+  end
+end

lib/pleroma/web/router.ex

@@ -217,6 +217,10 @@ defmodule Pleroma.Web.Router do
     plug(Pleroma.Web.Plugs.MappedSignatureToIdentityPlug)
   end
 
+  pipeline :inbox_guard do
+    plug(Pleroma.Web.Plugs.InboxGuardPlug)
+  end
+
   pipeline :static_fe do
     plug(Pleroma.Web.Plugs.StaticFEPlug)
   end
@@ -920,7 +924,7 @@ defmodule Pleroma.Web.Router do
   end
 
   scope "/", Pleroma.Web.ActivityPub do
-    pipe_through(:activitypub)
+    pipe_through([:activitypub, :inbox_guard])
     post("/inbox", ActivityPubController, :inbox)
     post("/users/:nickname/inbox", ActivityPubController, :inbox)
   end

test/pleroma/web/activity_pub/activity_pub_controller_test.exs

@@ -700,7 +700,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
       |> assign(:valid_signature, false)
       |> put_req_header("content-type", "application/activity+json")
       |> post("/inbox", params)
-      |> json_response(200)
+      |> json_response(400)
 
       assert all_enqueued() == []
     end