ISO 27001:2022 Annex A 5.25 Assessment and Decision on Information Security Events Explained

In this guide you will learn how to implement ISO 27001 Annex A 5.25 Assessment and decision on information security events and pass your audit from ISO 27001 Lead Auditor Stuart Barker – author of the ultimate ISO 27001 Toolkit.

ISO 27001 Annex A 5.25 is an ISO 27001 control that requires an organisation to assess information security events to categorise and prioritise them.

Purpose & Definition

The purpose of ISO 27001 Annex A 5.25 is a detective control that ensures effective categorisation and prioritisation of information security events.

The ISO 27001 standard defines ISO 27001 Annex A 5.25 as:

The organisation should assess information security events and decide if they are to be categorised as information security incidents.

ISO/IEC 27001:2022 Annex A 5.25 Assessment And Decision On Information Security Events

FREE ISO 27001 Annex A 5.25 Training Video

In this free training video you will learn How to implement ISO 27001 Assessment and Decision on Information Security Events (Annex A 5.25).

Implementation Guide

An information security incident is an event that could potentially have a negative impact on the confidentiality, integrity, or availability of information. The consequences of an incident can vary depending on the nature of the incident, the systems and data affected, and the organisation’s response.

The priority of an incident is determined by its impact and urgency. Impact refers to how severe the consequences of the incident are, and urgency refers to how quickly action is needed to resolve the incident.

Criteria for Categorising Events as Information Security Incidents

In roles and responsible ISO 27001 Annex A 5.24 Information Security Incident Management Planning And Preparation we assigned the various roles including the contact who will assess each information security incident using the agreed criteria.

The following can be used to categorise events as information security incidents:

• A virus
• Unauthorised access to information systems or data
• Ransomeware
• A system outage
• Social engineering attacks
• Unauthorised disclosure of information
• Unauthorised modification of information
• Denial of service to information systems or users

Assessment of Information Security Events

We also set out in ISO 27001 Annex A 5.24 Information Security Incident Management Planning And Preparation who will co-ordinate and respond to the information security incident and it those people that will perform the assessment and make a decision on the events.

The point of contact should assess each information security event using the following as guidance:

• Impact: The impact of the event should be assessed in terms of the severity of the consequences, such as loss of confidentiality, integrity, or availability of information.
• Urgency: The urgency of the event should be assessed in terms of how quickly a resolution is needed
• Priority: The priority of the event should be determined by its impact and urgency.

Information Security Assessment Formula

Impact x Urgency = Priority

Decision on Information Security Events

The point of contact should make a decision on each information security event based on the assessment. The decision should include the following:

• Whether the event is an information security incident
• The priority of the incident
• The appropriate response to the incident

Recording of Results

The results of the assessment and decision should be recorded. This is for reference, verification, lessons learnt, root cause analysis and continual improvement. We would record the following information:

• The date and time of the event
• The nature of the event
• The impact of the event
• The urgency of the event
• The priority of the event
• The decision on the event
• The response to the event

Implementation Conclusion

This guide provides a framework for identifying the impact, consequences and priority of an information security incident. The guide should be used in conjunction with your information security incident management plan.

The standard that relates to information security management for further reading if required is ISO/IEC 27035

How to implement ISO 27001 Annex A 5.25

Implementing Annex A 5.25 requires a structured approach to distinguish between routine events and genuine security incidents. As a Lead Auditor, I recommend following these ten steps to ensure your organisation responds with precision and consistency, maintaining the integrity of your Information Security Management System (ISMS).

1. Appoint and Define Assessment Roles

Provision clear accountability by identifying the specific individuals or teams responsible for evaluating information security events. This ensures that when an event occurs, there is no ambiguity regarding who makes the final decision.

• Assign a Primary Incident Manager or Point of Contact.
• Define IAM (Identity and Access Management) roles specifically for incident logging and assessment.
• Document these responsibilities within your Information Security Policy.

2. Establish Formal Assessment Criteria

Formalise the logic used to determine if an event qualifies as an incident. This creates an objective framework that removes guesswork and ensures consistency across different shifts or departments.

• Define “Event” versus “Incident” based on potential impact.
• Categorise events by type: such as system malfunctions, human error, or malicious attacks.
• Use a risk-based approach aligned with your organisational risk appetite.

3. Provision an Updated Asset Register

Integrate your Asset Register into the assessment workflow to provide critical context. Knowing the value and sensitivity of the affected asset allows for a more accurate assessment of the event’s severity.

• Ensure all technical assets are mapped to owners.
• Classify assets by criticality (e.g., High, Medium, Low).
• Reference the Asset Register during the initial triage phase.

4. Deploy a Centralised Event Logging Mechanism

Implement a technical solution to capture all reported events in a single, secure location. Centralisation prevents data loss and allows for chronological analysis of security trends.

• Utilise a SIEM (Security Information and Event Management) tool or a dedicated GRC platform.
• Ensure logs are immutable and time-stamped.
• Enable MFA (Multi-Factor Authentication) for all users accessing the log.

5. Implement a Triage Workflow

Create a step-by-step process for moving an event from “reported” to “assessed.” A formalised workflow ensures that high-priority events are prioritised over routine maintenance alerts.

• Set internal SLAs (Service Level Agreements) for assessment times.
• Use a ticketing system to track the status of each event.
• Define escalation paths for events that appear complex or high-risk.

6. Formalise the Decision Logic and Matrix

Develop a decision matrix that combines impact and urgency to produce a definitive classification. This matrix serves as the primary tool for the assessor to decide the next course of action.

• Map potential outcomes to Annex A 5.24 (Incident Management).
• Include “false positive” as a valid assessment result.
• Document the rationale for each decision within the event record.

7. Secure the Evidence Trail

Maintain a detailed record of the assessment process to satisfy audit requirements. This evidence proves that the control is operating effectively and that decisions are based on data rather than intuition.

• Capture snapshots of system logs or error messages.
• Record the identity of the assessor and the time the decision was reached.
• Store evidence in accordance with your document retention policy.

8. Conduct Competency and Awareness Training

Train all relevant personnel on the assessment criteria and the use of reporting tools. A technical control is only as effective as the people operating it: human error is a significant risk in event misclassification.

• Perform “Tabletop Exercises” to test the assessment logic.
• Brief staff on identifying social engineering or sophisticated phishing events.
• Include Annex A 5.25 requirements in new starter inductions.

9. Integrate with Incident Response Procedures

Ensure a seamless hand-off between event assessment and incident response. If an event is classified as an incident, the transition to the response phase must be immediate to minimize impact.

• Link the event log directly to the incident report.
• Notify the Incident Response Team automatically upon a “Positive” assessment.
• Define “Rules of Engagement” (ROE) for technical investigators.

10. Audit and Refine the Assessment Process

Review the history of assessments regularly to identify patterns or flaws in the criteria. Continual improvement ensures the process evolves alongside new security threats and organisational changes.

• Conduct a monthly review of “False Positives” to tune monitoring filters.
• Audit the event log for completeness and accuracy.
• Update the assessment matrix based on lessons learned from past incidents.

How to comply

To comply with ISO 27001 Annex A 5.25 you are going to implement the ‘how’ to the ‘what’ the control is expecting. In short measure you are going to:

1. Identify information security incidents.
2. Assess the impact of information security incidents.
3. Prioritise information security incidents.
4. Respond to information security incidents.
5. Record information security incidents.

How to pass the audit

To pass an audit of ISO 27001 Annex A 5.25 Assessment and decision on information security events you are going to make sure that you have followed the steps above in how to comply.

1. Have a documented information security incident management plan.
2. Implement the information security incident management plan.
3. Monitor the effectiveness of the information security incident management plan.
4. Review and update the information security incident management plan as needed.

What the auditor will check

The audit is going to check a number of areas. Lets go through the main ones

1. That you have documented your roles, responsibilities and process

The audit will check the documentation, that you have reviewed it and signed and it off and that it represents what you actually do not what you think they want to hear.

2. That you can demonstrate the process working

They are going to ask you for evidence to the incident management process and take one example. For this example you are going to show them and walk them through the process and prove that you followed it and that the process worked.

3. That you can learn your lesson

Documenting your lessons learnt and following this through to continual improvements or incident and corrective actions will be checked. They want to see that not only did you respond but that you learnt from it and did something to improve that reduced or eliminated the possibility of it happening again.

Top 3 Mistakes People Make and How to Avoid Them

The top 3 Mistakes People Make For ISO 27001 Annex A 5.25 are

1. Not having a documented information security incident management plan.

This is the most common mistake made by organisations. A documented information security incident management plan is essential for effective incident response. It should include the following:

• A process for identifying information security incidents.
• A process for assessing the impact of information security incidents.
• A process for prioritising information security incidents.
• A process for responding to information security incidents.
• A process for recording information security incidents.

2. Not implementing the information security incident management plan.

Even if you have a documented information security incident management plan, it is not enough to simply have the plan. The plan must be implemented in order to be effective. This means assigning responsibility for implementing the plan, providing training on the plan, and testing the plan.

3. Not monitoring the effectiveness of the information security incident management plan.

Once the information security incident management plan is implemented, it is important to monitor its effectiveness. This means reviewing reports of information security incidents, conducting audits of the plan, and taking corrective action as needed.

By avoiding these mistakes, you can ensure that you have an effective information security incident management plan in place.

ISO 27001 Annex A 5.25 FAQ

Related ISO 27001 Controls and Further Reading

• ISO 27001 Annex A 6.8 Information Security Event Reporting
• ISO 27001 Annex A 8.15 Logging
• ISO 27001 Annex A 5.26 Response To Information Security Incidents
• ISO 27001 Logging and Monitoring Policy Explained

ISO 27001 controls and attribute values

Control type	Information security properties	Cybersecurity concepts	Operational capabilities	Security domains
Detective	Confidentiality	Respond	Information Security Event Management	Defence
	Integrity	Detect
	Availability