  Watermark has been widely deployed by industry to detect AI-generated images.
The robustness of such watermark-based detector against evasion attacks in the
white-box and black-box settings is well understood in the literature. However,
the robustness in the no-box setting is much less understood. In particular,
multiple studies claimed that image watermark is robust in such setting. In
this work, we propose a new transfer evasion attack to image watermark in the
no-box setting. Our transfer attack adds a perturbation to a watermarked image
to evade multiple surrogate watermarking models trained by the attacker itself,
and the perturbed watermarked image also evades the target watermarking model.
Our major contribution is to show that, both theoretically and empirically,
watermark-based AI-generated image detector is not robust to evasion attacks
even if the attacker does not have access to the watermarking model nor the
detection API.
