  As the number of parameters in Deep Neural Networks (DNNs) scales, the thirst
for training data also increases. To save costs, it has become common for users
and enterprises to delegate time-consuming data collection to third parties.
Unfortunately, recent research has shown that this practice raises the risk of
DNNs being exposed to backdoor attacks. Specifically, an attacker can
maliciously control the behavior of a trained model by poisoning a small
portion of the training data. In this study, we focus on improving the
poisoning efficiency of backdoor attacks from the sample selection perspective.
The existing attack methods construct such poisoned samples by randomly
selecting some clean data from the benign set and then embedding a trigger into
them. However, this random selection strategy ignores that each sample may
contribute differently to the backdoor injection, thereby reducing the
poisoning efficiency. To address the above problem, a new selection strategy
named Improved Filtering and Updating Strategy (FUS++) is proposed.
Specifically, we adopt the forgetting events of the samples to indicate the
contribution of different poisoned samples and use the curvature of the loss
surface to analyses the effectiveness of this phenomenon. Accordingly, we
combine forgetting events and curvature of different samples to conduct a
simple yet efficient sample selection strategy. The experimental results on
image classification (CIFAR-10, CIFAR-100, ImageNet-10), text classification
(AG News), audio classification (ESC-50), and age regression (Facial Age)
consistently demonstrate the effectiveness of the proposed strategy: the attack
performance using FUS++ is significantly higher than that using random
selection for the same poisoning ratio.
