ISO 27001 A.5.25: Assessment and decision on information security events

Create clear assessment and classification criteria. Build a decision tree or flowchart that guides analysts through triage. Consider: type of event (unauthorised access, malware, data exposure), scope and scale, affected systems and data, potential business impact, and evidence of malicious intent. Centralise event collection and assessment. Use a SIEM to aggregate events from across your environment. Designate a SOC team or specific analysts responsible for initial triage. Define how quickly events at different severity levels need to be assessed. Build a classification matrix that maps event characteristics to incident severity levels. For each severity, define who gets notified, what response actions are required, and what timeframes apply. Make sure this aligns with your incident response plan and regulatory obligations. Train everyone involved in triage on the criteria and decision-making process. Run regular calibration exercises where the team reviews sample events and discusses how they would classify them. This builds consistency. Document all classification decisions and the reasoning behind them - auditors will want to see this. Close the feedback loop. If events are consistently misclassified or missed, update your detection rules, assessment criteria, and training. Use post-incident reviews to check whether initial triage was accurate and timely.