PixelDefend: Leveraging Generative Models to Understand and Defend against Adversarial Examples

TL;DR

PixelDefend leverages generative models to detect and purify adversarial examples by moving images back towards the training data distribution, significantly improving robustness of classifiers against various attacks.

Contribution

The paper introduces PixelDefend, a novel method that uses neural density models to detect and defend against adversarial examples by purification, independent of the classifier or attack type.

Findings

Detects adversarial perturbations effectively using density models

Significantly increases classification accuracy under attack

Works across multiple datasets and attack methods

Abstract

Adversarial perturbations of normal images are usually imperceptible to humans, but they can seriously confuse state-of-the-art machine learning models. What makes them so special in the eyes of image classifiers? In this paper, we show empirically that adversarial examples mainly lie in the low probability regions of the training distribution, regardless of attack types and targeted models. Using statistical hypothesis testing, we find that modern neural density models are surprisingly good at detecting imperceptible image perturbations. Based on this discovery, we devised PixelDefend, a new approach that purifies a maliciously perturbed image by moving it back towards the distribution seen in the training data. The purified image is then run through an unmodified classifier, making our method agnostic to both the classifier and the attacking method. As a result, PixelDefend can be used to protect already deployed models and be combined with other model-specific defenses. Experiments show that our method greatly improves resilience across a wide variety of state-of-the-art attacking methods, increasing accuracy on the strongest attack from 63% to 84% for Fashion MNIST and from 32% to 70% for CIFAR-10.