ASAP: Fast Adaptive Sliding Agnostic Poisoning Attack on Federated Learning

Federated Learning (FL) is vulnerable to model poisoning attacks, where malicious clients manipulate uploaded model updates to corrupt the global training process. Existing attacks are typically evaluated by their eventual damage, such as final accuracy degradation or convergence to random-guess performance, where the learned model becomes unusable and may ultimately lead to denial-of-service (DoS). However, such metrics overlook attack round complexity, defined as the number of communication rounds required for the poisoned global model to reach and remain near a desired degradation objective. In realistic FL systems, requiring more communication rounds increases the adversary's exposure to client participation, robust aggregation, filtering, and detection. We propose ASAP (Adaptive Sliding Agnostic Poisoning), a fast aggregation-rule-agnostic (AGR-agnostic) attack that treats the whole FL training process as an uncertain dynamical system and formulates poisoning as a feedback-control problem for steering the model toward a desired attack objective. ASAP combines adaptive sliding mode control (ASMC) with finite Fourier-basis uncertainty estimation, treating the unknown effects of benign training and aggregation as a time-varying uncertainty and using the resulting estimate to design the malicious updates. We theoretically prove that the sliding variable reaches the sliding surface in finite time and the tracking error subsequently converges exponentially. Experiments across multiple datasets, models, and aggregation rules demonstrate that ASAP reaches specified degradation objectives in fewer communication rounds while maintaining smaller target deviation than existing model poisoning attacks.