Adversarial machine learning defenses have primarily been focused on mitigating static, white-box attacks. However, it remains an open question whether such defenses are robust under an adaptive black-box adversary. In this paper, we specifically focus on the black-box threat model and make the following contributions: First we develop an enhanced adaptive black-box attack which is experimentally shown to be ≥ 30% more effective than the original adaptive black-box attack proposed by Papernot et al. For our second contribution, we test 10 recent defenses using our new attack and propose our own black-box defense (barrier zones). We show that our defense based on barrier zones offers significant improvements in security over state-of-the-art defenses. This improvement includes greater than 85% robust accuracy against black-box boundary attacks, transfer attacks and our new adaptive black-box attack, for the datasets we study. For completeness, we verify our claims through extensive experimentation with 10 other defenses using three adversarial models (14 different black-box attacks) on two datasets (CIFAR-10 and Fashion-MNIST).
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eBay Inc., San Jose, CA, USA , IBM Research, Thomas J. Watson Research Center, USA , Amazon Inc., Seattle, WA, USA
doi.org/10.1109/ACCESS.2021.3138966
IEEE Access
Centrum Wiskunde & Informatica, Amsterdam, The Netherlands

Mahmood, K, Nguyen, P.H, Nguyen, L.M, Nguyen, T, & van Dijk, M.E. (2021). Besting the black-box: Barrier zones for adversarial example defense. IEEE Access, 10, 1451–1474. doi:10.1109/ACCESS.2021.3138966