filecopyfallback in gio/gfile.c in GNOME GLib 2.15.0 through 2.61.1 creates new files with default permissions and set the correct permissions after the operation is finished. This might cause that the files can be accessible by more users during the operation than expected.
Upstream Commit:
https://gitlab.gnome.org/GNOME/glib/commit/d8f8f4d637ce43f8699ba94c9b7648beda0ca174