Res-MIA: A Training-Free Resolution-Based Membership Inference Attack on Federated Learning Models
This work addresses the privacy risk in federated learning where black-box models may still leak membership information about training data. The authors propose a training-free black-box membership inference attack that exploits differences in model sensitivity to high-frequency details induced by varying input resolutions. By applying controlled downsampling and upsampling operations to simulate resolution degradation, the method analyzes the resulting divergence in prediction confidence decay between member and non-member samples to infer membership status. Notably, the attack requires neither shadow models, auxiliary datasets, nor additional training—only a small number of forward queries. Evaluated on a federated ResNet-18 trained on CIFAR-10, the approach achieves an AUC of 0.88, substantially outperforming existing training-free baselines while incurring minimal computational overhead.