A Spectral View of Randomized Smoothing under Common Corruptions: Benchmarking and Improving Certified Robustness
"Certified robustness guarantee gauges a model’s resistance to test-time attacks and can assess the model’s readiness for deployment in the real world. In this work, we explore a new problem setting to critically examine how the adversarial robustness guarantees change when state-of-the-art randomized smoothing-based certifications encounter common corruptions of the test data. Our analysis demonstrates a previously unknown vulnerability of these certifiably robust models to low-frequency corruptions such as weather changes, rendering these models unfit for deployment in the wild. To alleviate this issue, we propose a novel data augmentation scheme, FourierMix, that produces augmentations to improve the spectral coverage of the training data. Furthermore, we propose a new regularizer that encourages consistent predictions on noise perturbations of the augmented data to improve the quality of the smoothed models. We show that FourierMix helps eliminate the spectral bias of certifiably robust models, enabling them to achieve significantly better certified robustness on a range of corruption benchmarks. Our evaluation also uncovers the inability of current corruption benchmarks to highlight the spectral biases of the models. To this end, we propose a comprehensive benchmarking suite that contains corruptions from different regions in the spectral domain. Evaluation of models trained with popular augmentation methods on the proposed suite unveils their spectral biases. It also establishes the superiority of FourierMix trained models in achieving stronger certified robustness guarantees under corruptions over the entire frequency spectrum."