Defending Poisoning Attacks in Federated Learning Under System and Data Heterogeneity
Federated learning (FL) is susceptible to poisoning attacks, where malicious clients manipulate local data or models to disrupt training. The system and data heterogeneity inherent in practical FL systems exacerbates these vulnerabilities, rendering existing defense mechanisms ineffective or infeasible. Specifically, distinguishing benign local models, trained on heterogeneous client data, from poisoned ones presents a significant challenge. Moreover, semi-asynchronous FL (SAFL) paradigms, commonly employed to address system heterogeneity, further complicate this issue by preventing fair evaluation of local models originating from different global models (i.e., with varying staleness). In this work, we propose a novel defensive framework (namely Fed-Beta) for robust and accurate FL model training under system and data heterogeneity. First, we introduce a staleness-aware SAFL paradigm, where the server accepts only a fixed number of local models per round and groups them based on their staleness. Then, we implement a two-stage aggregation mechanism. Specifically, we develop a robust intra-group aggregation method using model inversion to evaluate data-domain discrepancies among clients. This method accurately identifies and excludes malicious local models from aggregation, producing a reliable representative model for each group. Moreover, we design a model-consistency-aware inter-group aggregation method, which selectively aggregates group representative models with consistent update directions to update the global model. Theoretically, we conduct rigorous convergence analysis of Fed-Beta, offering insights into how system and data heterogeneity affect the defensive performance. Empirically, extensive experiments corroborate its superiority over existing schemes.