Title: The NTK Adversary: An Approach to Adversarial Attacks without any Model Access. Abstract: Adversarial attacks carefully perturb natural inputs, so that a machine learning algorithm produces erroneous decisions on them. Most successful attacks on neural networks exploit gradient information of the model (either directly or by estimating it through querying the model). Harnessing recent advances in Deep Learning theory, we propose a radically different attack that eliminates that need. In particular, in the regime where the Neural Tangent Kernel theory holds, we derive a simple, but powerful strategy for attacking models, which in contrast to prior work, does not require any access to the model under attack, or any trained replica of it for that matter. Instead, we leverage the explicit description afforded by the NTK to maximally perturb the output of the model, using solely information about the model structure and the training data. We experimentally verify the efficacy of our approach, first on models that lie close to the theoretical assumptions (large width, proper initialization, etc.) and, further, on more practical scenarios, with those assumptions relaxed. In addition,  we show that our perturbations exhibit strong transferability between models.