ACORN-IDS: Adaptive Continual Novelty Detection for Intrusion Detection Systems

Abstract: Intrusion Detection Systems (IDS) must maintain reliable detection performance under rapidly evolving benign traffic patterns and the continual emergence of cyberattacks, including zero-day threats with no labeled data available. However, most machine learning-based IDS approaches either assume static data distributions or rely on labeled attack samples, substantially limiting their applicability in real-world deployments. This setting naturally motivates continual novelty detection, which enables IDS models to incrementally adapt to non-stationary data streams without labeled attack data. In this work, we introduce ACORN-IDS, an adaptive continual novelty detection framework that learns exclusively from normal data while exploiting the inherent structure of an evolving unlabeled data stream. ACORN-IDS integrates a continual feature extractor, trained using reconstruction and metric learning objectives with clustering-based pseudo-labels, alongside a PCA-based reconstruction module for anomaly scoring. This design allows ACORN-IDS to continuously adapt to distributional shifts in both benign and malicious traffic. We conduct an extensive evaluation of ACORN-IDS on five realistic intrusion datasets under two continual learning scenarios: (i) Evolving Attacks and (ii) Evolving Normal and Attack Distributions. ACORN-IDS achieves, on average, a 62% improvement in F1-score and a 58% improvement in zero-day attack detection over the state-of-the-art unsupervised continual learning baseline. It also outperforms existing state-of-the-art novelty detection approaches while exhibiting near-zero forgetting and imposing minimal inference overhead. These results demonstrate that ACORN-IDS offers a practical, label-efficient solution for building adaptive and robust IDS in dynamic, real-world environments. We plan to release the code upon acceptance.