  Adversarial examples are typically constructed by perturbing an existing data
point within a small matrix norm, and current defense methods are focused on
guarding against this type of attack. In this paper, we propose unrestricted
adversarial examples, a new threat model where the attackers are not restricted
to small norm-bounded perturbations. Different from perturbation-based attacks,
we propose to synthesize unrestricted adversarial examples entirely from
scratch using conditional generative models. Specifically, we first train an
Auxiliary Classifier Generative Adversarial Network (AC-GAN) to model the
class-conditional distribution over data samples. Then, conditioned on a
desired class, we search over the AC-GAN latent space to find images that are
likely under the generative model and are misclassified by a target classifier.
We demonstrate through human evaluation that unrestricted adversarial examples
generated this way are legitimate and belong to the desired class. Our
empirical results on the MNIST, SVHN, and CelebA datasets show that
unrestricted adversarial examples can bypass strong adversarial training and
certified defense methods designed for traditional adversarial attacks.
