Paper:
PIXELDEFEND: LEVERAGING GENERATIVE MODELS TO UNDERSTAND AND DEFEND AGAINST ADVERSARIAL EXAMPLES
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.

Review:
The authors propose to use a generative model of images to detect and defend against adverarial examples. White-box attacks against standard models for image recognition (Resnet and VGG) are considered, and a generative model (a PixelCNN) is trained on the same data as the classifiers. The authors first show that adversarial examples created by the white-box attacks correspond to low likelihood region (according to the pixelCNN), which first gives a classification rule for detecting adversarial examples.
Then, to turn the genrative model into a defensive algorithm, the authors propose to preprocess test images by approximately maximizing the likelihood under similar constraints as the attacker of images, to "project" adversarial examples back to high-density regions (as estimated by the generative model). As a heuristic method, the authors propose to greedily maximize the likelihood of the incoming images pixel-by-pixel, which is possible because of the specific form of the PixelCNN likelihood in the context of l-infty attacks. An "adaptive" version of the algorithm, in which the preprocessing is used only when the likelihood of an example is below a certain threshold, is also proposed.
Experiments are carried out on Fashion MNIST and CIFAR-10. At a high level, the message is that projecting the image into a high density region is sufficient to correct for a significant portions of the mistakes made on adversarial examples. The main result is that this approach based on generative models seems to work even on against the strongest attacks.
Overall, the idea proposed in the paper, using a generative model to detect and filter out spurious patterns that can appear in adversarial examples, is rather intuitive. The experimental result that adversarial examples can somehow be corrected by a generative model is also interesting. The design choice of PixelCNN, which allows for a greedy optimization seems reasonable in that setting.
Whereas the paper is an interesting step forward, the paper still doesn't provide definitive arguments in favor of using such approaches in practice. There is a significant loss in accuracy on clean examples (2% on CIFAR-10 for a resnet), and more generally against weaker opponents such as the fast gradient sign. Thus, in reality, the experiments show that the pipeline generative model + classifier is robust against the strongest white box methods for this classifier, but on the other hand these methods do not transfer well to new models. This somewhat weakens the result, since robustness against these methods that do not transfer well is achieved by changing the model.