Tri-Level Network Attack Risk Stratification Using IDS-Contextual Ensemble Learning
Advanced Persistent Threats (APTs) remain among the most damaging risks to enterprise networks: they evade signature-based defenses, persist inside compromised environments, and cause substantial harm. Most machine learning approaches to intrusion detection treat the problem as binary classification and therefore offer little support for the risk prioritisation that Security Operations Centre (SOC) analysts actually require. This paper presents an Intrusion Detection System (IDS)-contextual ensemble learning framework that assigns network traffic to three operationally meaningful risk tiers: High, Medium and Low. Sixteen domain-knowledge engineered features are combined with IDS-generated contextual annotations, raw behavioural flow measurements and temporal components to form a 36-dimensional input representation. A one-way analysis of variance (ANOVA) with eta-squared effect sizing, performed before any model is trained, shows that none of the nine raw behavioural features carries meaningful discriminating power for risk-tier assignment, which establishes the need for IDS context. Five classifiers are evaluated on a balanced dataset of 125,000 APT flow records. Extra Trees performs best, reaching 89.35% accuracy, a Macro F1 of 0.8941, a Matthews Correlation Coefficient of 0.8404 and a macro Area Under the Curve (AUC) of 0.9829, at an inference cost of approximately 42 microseconds per record. A six-configuration ablation study shows that removing the IDS annotation alone reduces Macro F1 from 0.8941 to 0.3341, the chance level for three balanced classes. Robustness is confirmed by five-fold stratified cross-validation and by a fairness audit across the four network protocol types present in the data.