$σ$-zero: Gradient-based Optimization of $\ell_0$-norm Adversarial Examples

Evaluating the adversarial robustness of deep networks to gradient-based attacks is challenging. While most attacks consider $\ell2$- and $\ell\infty$-norm constraints to craft input perturbations, only a few investigate sparse $\ell1$- and $\ell0$-norm attacks. In particular, $\ell0$-norm attacks remain the least studied due to the inherent complexity of optimizing over a non-convex and non-differentiable constraint. However, evaluating adversarial robustness under these attacks could reveal weaknesses otherwise left untested with more conventional $\ell2$- and $\ell\infty$-norm attacks. In this work, we propose a novel $\ell0$-norm attack, called $\sigma$-zero, which leverages an ad hoc differentiable approximation of the $\ell0$ norm to facilitate gradient-based optimization, and an adaptive projection operator to dynamically adjust the trade-off between loss minimization and perturbation sparsity. Extensive evaluations using MNIST, CIFAR10, and ImageNet datasets, involving robust and non-robust models, show that $\sigma$-zero finds minimum $\ell0$-norm adversarial examples without requiring any time-consuming hyperparameter tuning, and that it outperforms all competing sparse attacks in terms of success rate, perturbation size, and scalability.