Containment: Never fetch locally

changelog.d/local-fetch-prevention.security

@@ -0,0 +1 @@
+Security: Block attempts to fetch activities from the local instance to prevent spoofing.

lib/pleroma/object/containment.ex

@@ -47,6 +47,19 @@ defmodule Pleroma.Object.Containment do
   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 whether an URL to fetch from is from the local server.
+
+  We never want to fetch from ourselves; if it's not in the database
+  it can't be authentic and must be a counterfeit.
+  """
+  def contain_local_fetch(id) do
+    case compare_uris(URI.parse(id), Pleroma.Web.Endpoint.struct_url()) do
+      :ok -> :error
+      _ -> :ok
+    end
+  end
+
   @doc """
   Checks that an imported AP object's actor matches the host it came from.
   """

lib/pleroma/object/fetcher.ex

@@ -148,6 +148,7 @@ defmodule Pleroma.Object.Fetcher do
 
     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, data} <- safe_json_decode(body),
          :ok <- Containment.contain_origin_from_id(id, data) do
@@ -160,6 +161,9 @@ defmodule Pleroma.Object.Fetcher do
       {:scheme, _} ->
         {:error, "Unsupported URI scheme"}
 
+      {:local_fetch, _} ->
+        {:error, "Trying to fetch local resource"}
+
       {:error, e} ->
         {:error, e}
 

test/pleroma/object/fetcher_test.exs

@@ -166,6 +166,13 @@ defmodule Pleroma.Object.FetcherTest do
                )
     end
 
+    test "it does not fetch from local instance" do
+      local_url = Pleroma.Web.Endpoint.url() <> "/objects/local_resource"
+
+      assert {:fetch, {:error, "Trying to fetch local resource"}} =
+               Fetcher.fetch_object_from_id(local_url)
+    end
+
     test "it validates content-type headers according to ActivityPub spec" do
       # Setup a mock for an object with invalid content-type
       mock(fn