ISO 27001:2022 Annex A Control 5.25

Explaining Annex A Control 5.25 Assessment and decision on information security events

ISO 27001 Annex A Control 5.25 - Assessment and Decision on Information Security Events in ISO 27001 focuses on ensuring that organizations systematically assess security events and determine whether they qualify as security incidents. The objective is to classify and prioritize events effectively, ensuring appropriate responses and documentation.

Control Type

Information Security Properties

Cybersecurity Concepts

Operational Capabilities

Security Domains

Objective of Control 5.25

The objective of ISO 27001 Control 5.25 is to define a structured process for assessing security events to determine whether they should be categorized as security incidents.

Your organization should aim to:

Establish clear criteria to distinguish between security events and incidents.
Assign responsibility for assessing and classifying security events.
Ensure that security teams can prioritize threats based on their potential impact.
Maintain documentation and reporting to support audits and compliance efforts.
Improve incident detection and response capabilities through structured assessments.

Purpose of Control 5.25

The purpose of this control is to:

Define a Standardized Approach
Ensure security events are evaluated consistently across different departments and teams.
Provide guidelines for categorization and prioritization to ensure effective incident response.
Minimize Security Risks
Identify high-risk security events early to prevent escalation.
Categorize events accurately so the most critical incidents receive immediate attention.
Support Compliance and Audits
Maintain detailed records of assessments to demonstrate compliance with ISO 27001, GDPR, NIST, and other security regulations.
Ensure security event logs can be referenced for forensic analysis.
Enhance Cybersecurity Posture
Establish a repeatable and scalable process to assess security events across all organizational units.
Ensure faster decision-making on security events, leading to improved incident response.

Implementing Control 5.25 in Your Organization

Your organization should develop and implement a structured process for assessing security events. This includes:

Developing a Categorization and Prioritization Scheme

A well-defined categorization framework should:

Establish clear criteria for identifying whether an event is a normal security event or a security incident.
Classify security events based on their:
- Type (e.g., malware detection, unauthorized access attempts, data exfiltration)
- Impact (e.g., minor anomaly vs. major system compromise)
- Likelihood of escalation (e.g., isolated event vs. part of a broader attack)

Example of a Security Event Classification Framework

Event Type	Severity Level	Action Required
Multiple failed login attempts from different locations	Medium	Investigate and monitor
Phishing email reported by employees	High	Quarantine email, educate staff, analyze email headers
Unusual data access patterns from an external source	Critical	Immediate response, escalate to incident team

Having a clear incident assessment flow ensures that security teams respond appropriately based on risk levels.

Assigning Responsibilities for Security Event Assessment

Define roles responsible for security event assessments, such as:

Security Analysts: Conduct initial assessments and categorize events.
Incident Response Team (IRT): Determines appropriate response actions.
Security Operations Center (SOC) Team: Continuously monitors and correlates event data.
Compliance and Audit Team: Ensures documentation meets regulatory requirements.
Create escalation paths to ensure security events are addressed at the right level.

Establishing an Event Assessment Workflow

A structured assessment workflow helps ensure security events are managed efficiently and consistently.

Security Event Assessment Process

Detection: Security monitoring tools (SIEM, IDS, antivirus) detect potential security events.
Initial Review: Security analysts investigate the event’s context and potential risks.
Categorization: Based on predefined criteria, the event is classified as:
- Non-critical event (e.g., false positive, minor policy violation).
- Potential incident requiring further investigation.
Escalation & Response: If classified as an incident, it is escalated to the incident response team.
Documentation & Reporting: Every decision is logged for compliance and future reference.

Ensuring Proper Documentation and Continuous Improvement

Maintain comprehensive records of all security event assessments and decisions. These records should include:

The nature of the event.
Criteria used for categorization and prioritization.
The decision-making process and responsible personnel.
Steps taken to mitigate risks and resolve the event.

Relevant ISO 27001 Controls

Control 5.25 is closely related to several other ISO 27001 controls. These include:

Control 5.24 Planning & Preparation to Security Incident Management: Supports the planning of security event management.
Control 5.26 Response to Security Incidents: Defines how your organization should respond to events classified as incidents.
Control 8.16 Monitoring of Networks and Systems: Focuses on real-time monitoring to detect anomalies and potential threats.

Templates and Tools to Support Control 5.25

To streamline the implementation of this control, consider using the following templates and tools:

Incident Response Policy Template: Outlines the roles, responsibilities, and procedures for handling security incidents.
Security Event Assessment Checklist: Provides a step-by-step guide for assessing and categorizing security events.
Incident Categorization and Prioritization Form: Helps document the criteria used to evaluate security events systematically.
ISO 27001 Risk Assessment Template: Assists in identifying and prioritizing risks associated with security events.

Additional Guidance

Organizations can further enhance their incident management strategy by aligning with ISO/IEC 27035, which provides:

Best practices for security event detection and classification.
Guidelines for escalation procedures.
Strategies for continuous improvement of incident response processes.