Design and evaluation of an Autonomous Cyber Defence agent using DRL and an augmented LLM

In this paper, we design and evaluate an Autonomous Cyber Defence (ACD) agent to monitor and act within critical network segments connected to untrusted infrastructure hosting active adversaries. We assume that modern network segments use software-defined controllers with the means to host ACD agents and other cybersecurity tools that implement hybrid AI models. Our agent uses a hybrid AI architecture that integrates deep reinforcement learning (DRL), augmented Large Language Models (LLMs), and rule-based systems. This architecture can be implemented in software-defined network controllers, enabling automated defensive actions such as monitoring, analysis, decoy deployment, service removal, and recovery. A core contribution of our work is the construction of three cybersecurity knowledge graphs that organise and map data from network logs, open source Cyber Threat Intelligence (CTI) reports, and vulnerability frameworks. These graphs enable automatic mapping of Common Vulnerabilities and Exposures (CVEs) to offensive tactics and techniques defined in the MITRE ATT&CK framework using Bidirectional Encoder Representations from Transformers (BERT) and Generative Pre-trained Transformer (GPT) models. Our experimental evaluation of the knowledge graphs shows that BERT-based models perform better, with precision (83.02%), recall (75.92%), and macro F1 scores (58.70%) significantly outperforming GPT models. The ACD agent was evaluated in a Cyber Operations Research (ACO) gym against eleven DRL models, including Proximal Policy Optimisation (PPO), Hierarchical PPO, and ensembles under two different attacker strategies. The results show that our ACD agent outperformed baseline implementations, with its DRL models effectively mitigating attacks and recovering compromised systems. In addition, we implemented and evaluated a chatbot using Retrieval-Augmented Generation (RAG) and a prompting agent augmented with the CTI reports represented in the cybersecurity knowledge graphs. The chatbot achieved high scores on generation metrics such as relevance (0.85), faithfulness (0.83), and semantic similarity (0.88), as well as retrieval metrics such as contextual precision (0.91). The experimental results suggest that the integration of hybrid AI systems with knowledge graphs can enable the automation and improve the precision of cyber defence operations, and also provide a robust interface for cybersecurity experts to interpret and respond to advanced cybersecurity threats.