What Is Incident Response

Incident response is the organized process of minimizing damage and rapidly recovering when a security incident occurs, such as unauthorized access, malware infection, data breach, or denial-of-service attack.

NIST SP 800-61 (Computer Security Incident Handling Guide) divides incident response into four phases: (1) Preparation, (2) Detection and Analysis, (3) Containment, Eradication, and Recovery, and (4) Post-Incident Activity.

Incidents are not a matter of "if" but "when." By developing a response plan in advance, organizing a CSIRT, and conducting regular drills, you can respond calmly and systematically when an actual incident occurs.

The 4 Phases of Incident Response

Phase 1: Preparation - Develop an incident response plan, assemble a CSIRT, establish communication channels, prepare tools, and conduct training. The quality of the preparation phase determines the success or failure of actual incident response.

Phase 2: Detection and Analysis - Detect incidents through alerts from SIEM and security monitoring tools, user reports, and notifications from external organizations. After detection, identify the scope of impact, determine severity, and organize the timeline. Preserving evidence from a digital forensics perspective is critical at this stage.

Phase 3: Containment, Eradication, and Recovery - Contain the damage (isolate infected endpoints from the network, disable compromised accounts), eradicate the threat (remove malware, patch vulnerabilities), and restore systems in a phased manner.

Phase 4: Post-Incident Activity - Analyze the root cause of the incident and develop measures to prevent recurrence. Conduct a lessons-learned review and incorporate improvements into the response plan and processes.

The Importance of Initial Response and Practical Tips

The most critical period in incident response is the first 72 hours. How you respond during this window significantly affects the scale of damage and recovery time.

Prioritize evidence preservation: In the rush to determine the cause, restarting systems or deleting logs destroys evidence needed for digital forensics. Capture memory dumps, create disk images, and back up logs first.

Activate the communication plan: Define in advance who to report to, when, and what to communicate. Reporting content and timing differ for each stakeholder, including executives, legal, public relations, customers, and regulatory authorities. In the case of a data breach, be mindful of notification deadlines under data protection laws such as GDPR.

Pre-define containment decision criteria: Decisions like "Should we disconnect the server from the network?" or "Should we shut down the service?" waste valuable time if debated from scratch during an incident. Define containment actions in advance as playbooks based on incident severity levels.

Developing and Drilling an Incident Response Plan

An Incident Response Plan should be documented with the following elements:

Incident definitions and classification criteria (severity levels)
CSIRT member composition and role assignments
Escalation flow and contact list
Response playbooks for each incident type (ransomware infection, unauthorized access, data breach, etc.)
Contact information for external partners (law enforcement, security vendors, attorneys)
Evidence preservation procedures
Templates for public relations and customer communications

Creating a plan alone is not enough. Conduct tabletop exercises at least once a year to verify the plan's effectiveness. During exercises, walk through response procedures based on a hypothetical incident scenario to identify gaps and areas for improvement.

Promptly incorporate issues discovered during exercises into the plan, and continuously improve response capabilities through the PDCA cycle.

To learn more about this topic, see What to Do After a Data Breach: A Step-by-Step Response Guide.

Common Misconceptions

An incident response plan is unnecessary if security products are in place: Security products help detect and prevent attacks, but 100% prevention is impossible. Without a response plan that defines who does what when a product fails to stop an incident, the response becomes ad hoc and damage escalates.
The first thing to do when an incident occurs is reboot the system: Rebooting destroys evidence in memory, including running processes, network connection information, and traces of malware. Preserve the evidence needed for forensic investigation before proceeding with containment and recovery.

Incident Response