Paper:
Adversarial attacks such as Gradient-based attacks, Fast Gradient Sign Method (FGSM) by Goodfellow et al. (2015) and DeepFool by Moosavi-Dezfooli et al. (2016) are input-dependent, small pixel-wise perturbations of images which fool state of the art neural networks into misclassifying images but are unlikely to fool any human. On the other hand a universal adversarial attack is an input-agnostic perturbation. The same perturbation is applied to all inputs and yet the neural network is fooled on a large fraction of the inputs. In this paper, we show that multiple known input-dependent pixel-wise perturbations share a common spectral property. Using this spectral property, we show that the top singular vector of input-dependent adversarial attack directions can be used as a very simple universal adversarial attack on neural networks. We evaluate the error rates and fooling rates of three universal attacks, SVD-Gradient, SVD-DeepFool and SVD-FGSM, on state of the art neural networks. We show that these universal attack vectors can be computed using a small sample of test inputs. We establish our results both theoretically and empirically. On VGG19 and VGG16, the fooling rate of SVD-DeepFool and SVD-Gradient perturbations constructed from observing less than 0.2% of the validation set of ImageNet is as good as the universal attack of Moosavi-Dezfooli et al. (2017a). To prove our theoretical results, we use matrix concentration inequalities and spectral perturbation bounds. For completeness, we also discuss another recent approach to universal adversarial perturbations based on (p, q)-singular vectors, proposed independently by Khrulkov & Oseledets (2018), and point out the simplicity and efficiency of our universal attack as the key difference.

Review:
This paper studied the problem of universal adversarial attack which is an input-agnostic perturbation. The authors proposed to use the top singular vector of input-dependent adversarial attack directions to perform universal adversarial attacks. The authors evaluated the error rates and fooling rates for three attacks on standard benchmark datasets.  
- The paper is generally well-written and easy to follow. My main concern towards this paper is about the experiments part from several aspects. First, the proposed method needs quite large L2 norm (50 on ImageNet) to work, while common adversarial attack experiments on ImageNet are usually conducted with L2 perturbation strength of 5 or less. I totally understand that performing universal attack would be much more difficult, yet having such loose L2 norm constraint still seems impractical. Second, the authors did not compare with any other baselines such as  (Moosavi-Dezfooli et al. 2017a) arguing that their universal attack is different for different perturbation strength and pixels are normalized. I do not think normalized pixel will be a problem as you can simply scale the perturbation strength accordingly. And because (Moosavi-Dezfooli et al. 2017a) uses different attack vectors for different perturbation strength, some comparison between these two types of universal attacks should be presented in order to mark the difference and demonstrate your advantages. I would suggest the authors to compare with several mentioned baselines in the paper to show the superiority of the proposed method.
- Theorem 1 seems interesting, yet it needs a special assumption. The authors argue that this is a reasonable assumption in a small neighborhood of x. I wonder if the authors could conduct some demonstrative experiments to verify this? Because the definition of S_x depends on the attack function, does it mean that the assumption need to be held for any attack function? Also regarding the choice of \delta, it seems that \delta is different for different x? If so, since u is also depend on \delta, this attack vector seems not universal?
Detailed comments:
- In proof of Theorem 1, all S should be G?
- In proof of Theorem 2, how to get \|v - \hat v\|_2 \leq \epsilon/(\gamma - \epsilon)? Directly applying the Theorems seems to get \epsilon / (\gamma) only?
Depending on whether the authors can address my concerns, I may change the final rating.
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after the rebuttal
I thank the authors for their response but I still feel that the assumption is not well-justified and there is still a lot to improve in terms of experiments. Therefore I decided to keep my score unchanged.