Incident Response Playbook for Large-Scale Compromise (Updated 2025)

This Large-Scale Compromise Incident Response Playbook provides a structured and detailed approach to managing widespread security incidents affecting multiple systems or services. This includes modern detection, containment, and recovery techniques integrated with organisational procedures to minimise damage and downtime.

1. Preparation

Objective: Establish protocols, tools, and communication strategies to handle large-scale security incidents effectively.

Key Steps:

1. Deploy and Configure Security Tools:

   • Use Endpoint Detection and Response (EDR) tools to detect, contain, and remediate threats on a large scale.

   • Implement SIEM systems to aggregate logs from various security tools for real-time analysis.

   • Ensure Threat Intelligence feeds are configured to detect Indicators of Compromise (IOCs).

2. Maintain and Update Network and Asset Documentation:

   • Keep an updated inventory of critical systems, applications, and network architecture.

   • Ensure network segmentation to prevent the rapid spread of malware or unauthorised access.

3. Develop Communication Plans:

   • Define internal and external communication strategies, including crisis management protocols.

   • Establish contacts with law enforcement, CERTs, and relevant regulatory bodies for timely escalation if necessary.

4. Baseline Traffic and System Behaviour:

   • Establish normal traffic flows and resource utilisation patterns to detect anomalies early.

   • Conduct regular security audits and penetration tests to identify weaknesses.

5. Staff Training and Awareness:

   • Provide incident response training to security teams.

   • Conduct simulations and tabletop exercises to test the organisation's readiness for large-scale incidents.

2. Identification

Objective: Detect the compromise, determine its scope, and notify the relevant stakeholders.

Detection Techniques:

1. Monitoring and Alerts:

   • Analyse alerts from EDR, SIEM, IDS/IPS, and antivirus tools for signs of compromise.

   • Monitor network traffic for connections to Command-and-Control (C2) servers, Tor gateways, or suspicious domains.

2. Investigate Initial Indicators:

   • Check for locked accounts, unusual logins, or high data transfer rates.

   • Search for binaries in system directories such as %APPDATA% or /tmp that may indicate malicious activity.

3. Identify the Scope:

   • Use forensic tools to trace the spread of the attack across the network.

   • Examine logs to determine if lateral movement, data exfiltration, or privilege escalation occurred.

4. External Notifications:

   • Notify abuse teams, law enforcement, and regulators if legally required.

3. Containment

Objective: Limit the attack’s impact and prevent further damage to the organisation’s infrastructure.

Steps:

1. Isolate Affected Areas:

   • Disconnect compromised systems from the network while preserving their state for forensic analysis.

   • Use VLAN segmentation to isolate infected areas and restrict external access.

2. Block Malicious Communication:

   • Implement firewall rules to block communication with C2 servers and other malicious endpoints.

   • Disable compromised or suspicious accounts.

3. Communicate with Stakeholders:

   • Activate a crisis management team to coordinate response efforts.

   • Provide clear instructions to employees regarding access restrictions and necessary security measures.

4. Monitor and Adjust:

   • Continuously monitor the containment area to ensure the threat does not spread to other parts of the network.

4. Remediation

Objective: Remove the threat from the environment and prevent recurrence.

Steps:

1. Eliminate Threat Components:

   • Identify and remove all malicious files, scripts, and processes.

   • Revoke access for compromised accounts and reset all affected credentials.

2. Harden Systems:

   • Apply patches and updates to vulnerable systems.

   • Reinforce security configurations, including Group Policy Objects (GPOs), firewall rules, and access controls.

3. Network-Wide Actions:

   • Review all communication channels for signs of backdoors or compromised connections.

   • Involve the legal and HR teams if the threat originates from internal actors.

4. Coordinate with Security Vendors:

   • Share newly identified IOCs with security providers to improve detection and prevention.

5. Recovery

Objective: Restore affected systems and services to full functionality.

Steps:

1. Verify System Integrity:

   • Conduct thorough scans to ensure no malicious files or processes remain.

   • Perform system reinstallation from trusted sources where necessary.

2. Restore Data:

   • Restore altered or deleted files from verified clean backups.

   • Implement strong password policies and enforce credential resets.

3. Reintegrate Systems:

   • Gradually reconnect isolated network segments, verifying that each step does not introduce further risk.

   • Apply geo-blocking and other traffic filters to prevent unauthorised access.

4. Test and Monitor:

   • Test all critical business processes to ensure systems are operating correctly.

   • Implement enhanced monitoring for early detection of potential re-infection.

6. Lessons Learned

Objective: Document the incident, evaluate response efforts, and improve organisational defences.

Steps:

1. Create an Incident Report:

   • Document the root cause, timeline of events, key actions taken, and outcomes.

   • Include lessons learned, such as detection gaps and areas for improvement.

2. Evaluate Performance:

   • Conduct a post-incident review with security teams, IT operations, and senior management.

   • Identify strengths and weaknesses in the response process.

3. Update Playbooks:

   • Revise incident response procedures to incorporate lessons learned.

   • Enhance detection rules, access controls, and monitoring capabilities.

4. Training and Awareness:

   • Provide targeted training for employees on improved security practices.

   • Conduct regular drills to test updated response capabilities.

Conclusion

This Large-Scale Compromise Incident Response Playbook equips organisations with a comprehensive framework to handle extensive security incidents effectively. Continuous updates to monitoring tools, playbooks, and staff training are critical to maintaining organisational resilience against large-scale threats.

For additional support, including forensic tool recommendations, communication templates, and recovery guides, contact your cybersecurity operations team.