  Image scaling is an integral part of machine learning and computer vision
systems. Unfortunately, this preprocessing step is vulnerable to so-called
image-scaling attacks where an attacker makes unnoticeable changes to an image
so that it becomes a new image after scaling. This opens up new ways for
attackers to control the prediction or to improve poisoning and backdoor
attacks. While effective techniques exist to prevent scaling attacks, their
detection has not been rigorously studied yet. Consequently, it is currently
not possible to reliably spot these attacks in practice.
  This paper presents the first in-depth systematization and analysis of
detection methods for image-scaling attacks. We identify two general detection
paradigms and derive novel methods from them that are simple in design yet
significantly outperform previous work. We demonstrate the efficacy of these
methods in a comprehensive evaluation with all major learning platforms and
scaling algorithms. First, we show that image-scaling attacks modifying the
entire scaled image can be reliably detected even under an adaptive adversary.
Second, we find that our methods provide strong detection performance even if
only minor parts of the image are manipulated. As a result, we can introduce a
novel protection layer against image-scaling attacks.
