Less is More: Persistent Low-Frequency Backdoor Injection in Federated Learning

Federated learning (FL) enables multiple clients to collaboratively train a machine learning model without sharing their local data. However, the distributed nature of FL makes it vulnerable to backdoor attacks from malicious clients. Most existing attack methods often assume that attackers can inject backdoors in every training round - a scenario that is both unrealistic and inefficient in real-world FL deployment. In this paper, we investigate why backdoor attacks become less effective under low-frequency injection and propose a novel attack paradigm for FL, called REinforced Memorization-based INterval backDoor attack (REMIND). REMIND optimizes the backdoor trigger via task alignment and feature alignment. Task alignment aligns backdoor and main task objectives to resist benign update suppression during non-attack rounds, while feature alignment guides poisoned samples to match the activation trajectory of target-class samples. This dual alignment enhances the backdoor's persistence and narrows the divergence between malicious and benign updates. With strong attack success rates established, we further analyze the advantages of low-frequency backdoor attacks, particularly their ability to improve robustness against defense mechanisms. Extensive evaluations on four benchmark datasets show that REMIND consistently outperforms eight state-of-the-art attack baselines under nine defense strategies.