4 files changed, 125 insertions, 2 deletions
diff --git a/lib/pleroma/constants.ex b/lib/pleroma/constants.ex
index 3a5e35301..5268ebe7a 100644
--- a/lib/pleroma/constants.ex
+++ b/lib/pleroma/constants.ex
@@ -85,6 +85,36 @@ defmodule Pleroma.Constants do
     ]
   )
 
+  const(activity_types,
+    do: [
+      "Create",
+      "Update",
+      "Delete",
+      "Follow",
+      "Accept",
+      "Reject",
+      "Add",
+      "Remove",
+      "Like",
+      "Announce",
+      "Undo",
+      "Flag",
+      "EmojiReact"
+    ]
+  )
+
+  const(allowed_activity_types_from_strangers,
+    do: [
+      "Block",
+      "Create",
+      "Flag",
+      "Follow",
+      "Like",
+      "EmojiReact",
+      "Announce"
+    ]
+  )
+
   # basic regex, just there to weed out potential mistakes
   # https://datatracker.ietf.org/doc/html/rfc2045#section-5.1
   const(mime_regex,
diff --git a/lib/pleroma/web/activity_pub/activity_pub_controller.ex b/lib/pleroma/web/activity_pub/activity_pub_controller.ex
index cdd054e1a..a08eda5f4 100644
--- a/lib/pleroma/web/activity_pub/activity_pub_controller.ex
+++ b/lib/pleroma/web/activity_pub/activity_pub_controller.ex
@@ -311,7 +311,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubController do
       post_inbox_relayed_create(conn, params)
     else
       conn
-      |> put_status(:bad_request)
+      |> put_status(403)
       |> json("Not federating")
     end
   end
diff --git a/lib/pleroma/web/plugs/inbox_guard_plug.ex b/lib/pleroma/web/plugs/inbox_guard_plug.ex
new file mode 100644
index 000000000..0064cce76
--- /dev/null
+++ b/lib/pleroma/web/plugs/inbox_guard_plug.ex
@@ -0,0 +1,89 @@
+# 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: [activity_types: 0, 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
+    with {_, true} <- {:federating, Config.get!([:instance, :federating])} do
+      conn
+      |> filter_activity_types()
+    else
+      {:federating, false} ->
+        conn
+        |> json(403, "Not federating")
+        |> halt()
+    end
+  end
+
+  def call(conn, _opts) do
+    with {_, true} <- {:federating, Config.get!([:instance, :federating])},
+         conn = filter_activity_types(conn),
+         {:known, true} <- {:known, known_actor?(conn)} do
+      conn
+    else
+      {:federating, false} ->
+        conn
+        |> json(403, "Not federating")
+        |> halt()
+
+      {:known, false} ->
+        conn
+        |> filter_from_strangers()
+    end
+  end
+
+  # Early rejection of unrecognized types
+  defp filter_activity_types(%{body_params: %{"type" => type}} = conn) do
+    with true <- type in activity_types() do
+      conn
+    else
+      _ ->
+        conn
+        |> json(400, "Invalid activity type")
+        |> halt()
+    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")
+        |> halt()
+    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
diff --git a/lib/pleroma/web/router.ex b/lib/pleroma/web/router.ex
index 6492e3861..9b9ee421c 100644
--- a/lib/pleroma/web/router.ex
+++ b/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