Title: EFFECTIVE FREQUENCY-BASED BACKDOOR ATTACKS WITH LOW POISONING RATIOS. Abstract: Backdoor attack has been considered a serious threat to deep learning. Although several seminal backdoor attack methods have been proposed, they often required at least a certain poisoning ratio (\eg, 1\% or more) to achieve high attack success rate (ASR). 
However, the attack with a large poisoning ratio may be difficult to evade human inspection or backdoor defenses, \ie, low stealthiness. 
To tackle the dilemma between high ASR and low stealthiness, we aim to enhance ASR under low poisoning ratio, \ie, pursuing high ASR and high stealthiness simultaneously. To achieve this goal, we propose a novel frequency-based backdoor attack, where the trigger is generated based on important frequencies that contribute positively to the model prediction with respect to the target class. 
Extensive experiments on four benchmark datasets (CIFAR-10, CIFAR-100, GTSRB, Tiny ImageNet) verify the effectiveness and stealthiness of the proposed method under extremely low poisoning ratios. Specifically, with only 0.01\% poisoning ratio, our attack could achieve the ASR of 80.51%, 51.3%, 76.3%, and 87.2% on above four datasets, respectively, while the ASR values of most state-of-the-art (SOTA) attack methods are close to 0. Meanwhile, our method could well evade several SOTA backdoor defense methods, \ie, the ASR values are not significantly affected under defense. 