  Deep neural networks have recently achieved tremendous success in image
classification. Recent studies have however shown that they are easily misled
into incorrect classification decisions by adversarial examples. Adversaries
can even craft attacks by querying the model in black-box settings, where no
information about the model is released except its final decision. Such
decision-based attacks usually require lots of queries, while real-world image
recognition systems might actually restrict the number of queries. In this
paper, we propose qFool, a novel decision-based attack algorithm that can
generate adversarial examples using a small number of queries. The qFool method
can drastically reduce the number of queries compared to previous
decision-based attacks while reaching the same quality of adversarial examples.
We also enhance our method by constraining adversarial perturbations in
low-frequency subspace, which can make qFool even more computationally
efficient. Altogether, we manage to fool commercial image recognition systems
with a small number of queries, which demonstrates the actual effectiveness of
our new algorithm in practice.
