StealEmojiPolicy: Sanitize shortcodes

Closes: https://git.pleroma.social/pleroma/pleroma/-/issues/3245

lib/pleroma/web/activity_pub/mrf/steal_emoji_policy.ex

@@ -34,6 +34,7 @@ defp steal_emoji({shortcode, url}, emoji_dir_path) do
           |> Path.basename()
           |> Path.extname()
 
+        shortcode = Path.basename(shortcode)
         file_path = Path.join(emoji_dir_path, shortcode <> (extension || ".png"))
 
         case File.write(file_path, response.body) do
@@ -76,6 +77,7 @@ def filter(%{"object" => %{"emoji" => foreign_emojis, "actor" => actor}} = messa
       new_emojis =
         foreign_emojis
         |> Enum.reject(fn {shortcode, _url} -> shortcode in installed_emoji end)
+        |> Enum.reject(fn {shortcode, _url} -> String.contains?(shortcode, ["/", "\\"]) end)
         |> Enum.filter(fn {shortcode, _url} ->
           reject_emoji? =
             [:mrf_steal_emoji, :rejected_shortcodes]

test/pleroma/web/activity_pub/mrf/steal_emoji_policy_test.exs

@@ -60,6 +60,32 @@ test "Steals emoji on unknown shortcode from allowed remote host", %{
            |> File.exists?()
   end
 
+  test "rejects invalid shortcodes", %{path: path} do
+    message = %{
+      "type" => "Create",
+      "object" => %{
+        "emoji" => [{"fired/fox", "https://example.org/emoji/firedfox"}],
+        "actor" => "https://example.org/users/admin"
+      }
+    }
+
+    fullpath = Path.join(path, "fired/fox.png")
+
+    Tesla.Mock.mock(fn %{method: :get, url: "https://example.org/emoji/firedfox"} ->
+      %Tesla.Env{status: 200, body: File.read!("test/fixtures/image.jpg")}
+    end)
+
+    clear_config(:mrf_steal_emoji, hosts: ["example.org"], size_limit: 284_468)
+
+    refute "firedfox" in installed()
+    refute File.exists?(path)
+
+    assert {:ok, _message} = StealEmojiPolicy.filter(message)
+
+    refute "fired/fox" in installed()
+    refute File.exists?(fullpath)
+  end
+
   test "reject regex shortcode", %{message: message} do
     refute "firedfox" in installed()