“…Currently, there are also some approaches to alleviate the backdoor threat. Existing defenses are mostly empirical, which can be divided into five main categories, including (1) detection-based defenses (Xu et al, 2021;Zeng et al, 2021a;Xiang et al, 2022), (2) preprocessing based defenses (Doan et al, 2020;Li et al, 2021b;Zeng et al, 2021b), (3) model reconstruction based defenses (Zhao et al, 2020a;Li et al, 2021a;Zeng et al, 2022), (4) trigger synthesis based defenses Dong et al, 2021;Shen et al, 2021), and (5) poison suppression based defenses Borgnia et al, 2021). Specifically, detection-based defenses examine whether a suspicious DNN or sample is attacked and it will deny the use of malicious objects; Preprocessing based methods intend to damage trigger patterns contained in attack samples to prevent backdoor activation by introducing a preprocessing module before feeding images into DNNs; Model reconstruction based ones aim at removing the hidden backdoor in DNNs by modifying models directly; The fourth type of defenses synthesize potential trigger patterns at first, following by the second stage that the hidden backdoor is eliminated by suppressing their effects; The last type of methods depress the effectiveness of poisoned samples during the training process to prevent the creation of hidden backdoors.…”