Robustness against image perturbations bounded by a $\ell_p$ ball have been
well-studied in recent literature. Perturbations in the real-world, however,
rarely exhibit the pixel independence that $\ell_p$ threat models assume. A
recently proposed Wasserstein distance-bounded threat model is a promising
alternative that limits the perturbation to pixel mass movements. We point out
and rectify flaws in previous definition of the Wasserstein threat model and
explore stronger attacks and defenses under our better-defined framework.
Lastly, we discuss the inability of current Wasserstein-robust models in
defending against perturbations seen in the real world. Our code and trained
models are available at https://github.com/edwardjhu/improved_wasserstein .