Scrub content-type of uploaded media before serving

changelog.d/3895.add

@@ -0,0 +1 @@
+Uploaded media content-type is scrubbed before serving the files to limit the security impact of some attachments

lib/pleroma/web/plugs/uploaded_media.ex

@@ -55,6 +55,7 @@ def call(%{request_path: <<"/", @path, "/", file::binary>>} = conn, opts) do
          {:ok, get_method} <- uploader.get_file(file),
          false <- media_is_banned(conn, get_method) do
       get_media(conn, get_method, proxy_remote, opts)
+      |> scrub_mime()
     else
       {:valid_host, false} ->
         redirect_url =
@@ -131,4 +132,26 @@ defp get_media(conn, unknown, _, _) do
     |> send_resp(:internal_server_error, dgettext("errors", "Internal Error"))
     |> halt()
   end
+
+  defp scrub_mime(%Plug.Conn{resp_headers: headers} = conn) do
+    [{_, mimetype}] = Enum.filter(headers, fn {x, _y} -> match?("content-type", x) end)
+
+    [_type, subtype] = String.split(mimetype, "/")
+
+    cond do
+      String.contains?(subtype, ["javascript", "ecmascript", "jscript"]) ->
+        force_plaintext(conn)
+
+      String.contains?(mimetype, ["text/html", "text/xml", "application/xml"]) ->
+        force_plaintext(conn)
+
+      true ->
+        conn
+    end
+  end
+
+  defp force_plaintext(conn) do
+    conn
+    |> merge_resp_headers([{"content-type", "text/plain"}])
+  end
 end

test/pleroma/web/plugs/uploaded_media_plug_test.exs

@@ -66,4 +66,33 @@ test "denies access to media if wrong Host", %{
 
     assert redirected_to(conn, 302) == expected_url
   end
+
+  test "Filters out dangerous content types" do
+    context = %{module: __MODULE__, case: __MODULE__}
+
+    test_files = [
+      "test/fixtures/lain.xml",
+      "test/fixtures/nypd-facial-recognition-children-teenagers.html",
+      "test/fixtures/snow.js"
+    ]
+
+    Enum.each(test_files, fn t ->
+      Pleroma.DataCase.ensure_local_uploader(context)
+      filename = String.split(t, "/") |> List.last()
+
+      upload = %Plug.Upload{
+        path: Path.absname(t),
+        filename: filename
+      }
+
+      {:ok, %{"url" => [%{"href" => attachment_url}]}} = Upload.store(upload)
+
+      conn = get(build_conn(), attachment_url)
+
+      assert Enum.any?(
+               conn.resp_headers,
+               &(&1 == {"content-type", "text/plain"})
+             )
+    end)
+  end
 end