ISO/IEC 27001:2022 Annex A · Control 5.25

Assessment and Decision on Information Security Events: A Practical Implementation Guide

Evaluate reported events consistently so genuine incidents receive the right urgency and ownership.

This control concerns assessing information security events and deciding whether they should be categorized as incidents.

Practical interpretation: Not every alert is an incident, but every meaningful event needs timely, documented triage using consistent criteria and available context.

What should the control achieve?

Events enter a defined assessment channel.
Severity and incident criteria are consistent.
Decisions are recorded and escalated promptly.
Related events can be correlated.

Step-by-step implementation

Define event and incident criteria

Use impact, scope, sensitivity, persistence, threat and legal implications.

Create intake channels

Centralize reports from people, monitoring, suppliers and customers.

Assign triage responsibility

Ensure trained analysts and on-call coverage.

Gather context

Check assets, users, logs, threat information and business impact.

Decide and escalate

Record classification, severity, rationale and next owner.

Review quality

Sample decisions and tune criteria from false positives and missed incidents.

What this could look like in practice

A DLP alert shows a customer file uploaded to personal cloud storage. Triage confirms sensitive data, external exposure and policy violation. The analyst records severity, declares an incident and activates privacy and management escalation.

Activity	Practical implementation	Evidence
User report	Service desk captures required facts and urgency.	Event ticket
Technical alert	Analyst enriches with asset and identity context.	Triage notes
Decision	Criteria determine event closure or incident activation.	Classification record

Implementation evidence

Event assessment procedure
Incident criteria
Severity matrix
Triage records
Escalation logs
Correlation rules
Quality reviews
Analyst training

Useful metrics

Time to triage
Events promoted to incidents
Reopened or misclassified events
Events breaching assessment target

Common mistakes

Treating tool severity as business severity.
Closing events without rationale.
No coverage outside office hours.
Ignoring low-level events that form a pattern.
Delaying legal or privacy escalation.

Questions an auditor may ask

How do you distinguish event from incident?
Show two different triage decisions.
How is business context obtained?
How are related events correlated?

Implementation test: Give multiple analysts the same scenarios and verify they reach comparable classifications and escalation decisions.

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This independent educational guide paraphrases the practical intent of the control and does not replace the official ISO standards, professional advice or an organization-specific risk assessment. It is not affiliated with or endorsed by ISO.

ISO 27001 Annex A 5.25: Assessment and Decision on Information Security Events – Practical Implementation Guide