Facilitating Early-Stage Backdoor Attacks in Federated Learning with Whole Population Distribution Inference

The development of the Internet of Things (IoT) combined with the emergence of federated learning (FL) makes it possible for mobile edge computing (MEC) to gain insight from physically separated data without violating privacy or burdening communication. Due to the distributed nature of MEC devices, researchers have uncovered that the FL is vulnerable to backdoor attacks, which aim at injecting a subtask into the FL without corrupting the performance of the main task. The backdoor attack achieves high accuracy on both the main task and the backdoor subtask when injected at FL model convergence. However, the effectiveness of the backdoor is weak when injected in early training stage. In this paper, we strengthen the early-injected backdoor attack by using information leakage. We show that FL convergence can be expedited if the clients dataset mimicks the distribution and gradients of the whole population. Based on this observation, we propose a two-phase backdoor attack, which includes a preliminary phase for the subsequent backdoor attack. Taking advantage of the preliminary phase, the later injected backdoor achieves better effectiveness, as the backdoor effect is less likely to be diluted by normal model updates. Extensive experiments are conducted on the MNIST dataset under various data heterogeneity settings to evaluate the effectiveness of the proposed backdoor attack. The results show that the proposed backdoor outperforms existing backdoor attacks in both success rate and longevity, even when defense mechanisms are in place.