HARP: Let Object Detector Undergo Hyperplasia to Counter Adversarial Patches

Adversarial patches can mislead object detectors to produce erroneous predictions. To defend against adversarial patches, one can take two types of protections on the model side, including modifying the detector itself (e.g., adversarial training) or attaching a new model in front of the detector. However, the former often deteriorates clean performance of detectors, and the latter may have high deployment costs caused by too many training parameters. Inspired by the phenomenon of "bone hyperplasia" in human bodies, we present a novel model-side adversarial patch defense, called HARP (Hyperplasia based Adversarial Patch defense). Just as bone hyperplasia can enhance bone strength and skeletal stability, the hyperostosia of detectors can also help to resist adversarial patches. Following this idea, HARP chooses to improve adversarial robustness by "growing" lightweight CNN modules (i.e., hyperplasia modules) on the pre-trained object detectors. We conduct extensive experiments on the PASCAL VOC and COCO datasets to compare HARP with the data-side defense JPEG and the model-side defenses adversarial training, SAC and FNC. Experimental results show that HARP provides excellent defense against adversarial patches while maintaining clean performance, outperforming the compared defense methods. Under PGD-based adaptive attacks, HARP surpasses the recently proposed defense method SAC by 12.5% in mean average precision (mAP) on PASCAL VOC, and 13.2% on COCO dataset. In addition, experiments confirm that the increase in model inference time caused by HARP is almost negligible.