The increasing deployment of Federated Learning (FL) in Intrusion Detection Systems (IDS) introduces new challenges related to data privacy, centralized coordination, and susceptibility to poisoning attacks. While significant research has focused on protecting traditional FL-IDS with centralized aggregation servers, there remains a notable gap in addressing the unique challenges of decentralized FL-IDS (DFL-IDS). This study aims to address the limitations of traditional centralized FL-IDS by proposing a novel defense framework tailored for the decentralized FL-IDS architecture, with a focus on privacy preservation and robustness against poisoning attacks. We propose PenTiDef, a privacy-preserving and robust defense framework for DFL-IDS, which incorporates Distributed Differential Privacy (DDP) to protect data confidentiality and utilizes latent space representations (LSR) derived from neural networks to detect malicious updates in the decentralized model aggregation context. To eliminate single points of failure and enhance trust without a centralized aggregation server, PenTiDef employs a blockchain-based decentralized coordination mechanism that manages model aggregation, tracks update history, and supports trust enforcement through smart contracts. Experimental results on CIC-IDS2018 and Edge-IIoTSet demonstrate that PenTiDef consistently outperforms existing defenses (e.g., FLARE, FedCC) across various attack scenarios and data distributions. These findings highlight the potential of PenTiDef as a scalable and secure framework for deploying DFL-based IDS in adversarial environments. By leveraging privacy protection, malicious behavior detection in hidden data, and working without a central server, it provides a useful security solution against real-world attacks from untrust participants.

Model extraction is a severe threat to Machine Learning-as-a-Service systems, especially through data-free approaches, where dishonest users can replicate the functionality of a black-box target model without access to realistic data. Despite recent advancements, existing data-free model extraction methods suffer from the oscillating accuracy of the substitute model. This oscillation, which could be attributed to the constant shift in the generated data distribution during the attack, makes the attack impractical since the optimal substitute model cannot be determined without access to the target model's in-distribution data. Hence, we propose MetaDFME, a novel data-free model extraction method that employs meta-learning in the generator training to reduce the distribution shift, aiming to mitigate the substitute model's accuracy oscillation. In detail, we train our generator to iteratively capture the meta-representations of the synthetic data during the attack. These meta-representations can be adapted with a few steps to produce data that facilitates the substitute model to learn from the target model while reducing the effect of distribution shifts. Our experiments on popular baseline image datasets, MNIST, SVHN, CIFAR-10, and CIFAR-100, demonstrate that MetaDFME outperforms the current state-of-the-art data-free model extraction method while exhibiting a more stable substitute model's accuracy during the attack.