VOID: Backdoor Injection through Knowledge Vacuity in Federated Unlearning

Federated unlearning (FU) enables federated systems to remove designated data from a trained global model, but its security risks remain poorly understood. We show that calibration-based FU introduces a structural vulnerability through finite-step post-hoc corrections, which leave behind knowledge vacuity in weakly constrained residual dimensions where the unlearned data's influence is suppressed while retained-task recovery pressure remains limited. We propose VOID, an unlearning-phase backdoor attack that exploits knowledge vacuity to implant trigger semantics along the legitimate unlearning trajectory. VOID identifies these residual dimensions through influence-based trajectory estimation and neuron-level vacuity profiling, then performs masked semantic substitution during unlearning. Across datasets and FU methods, VOID achieves up to 99% attack success, preserves clean accuracy, and persists after post-unlearning finetuning. Our results show that approximate forgetting can expose writable capacity for adversarial reuse.