A company runs a web application on an Auto Scaling group of EC2 instances behind an Application Load Balancer. The application stores user session data in an ElastiCache Redis cluster. The security team receives an alert from GuardDuty that one of the EC2 instances is communicating with a known command-and-control (C2) IP address. The instance ID is i-0a1b2c3d4e5f. The security engineer needs to contain the threat immediately while preserving the instance for forensic analysis. Which course of action should the security engineer take?
Applying a new security group that denies all inbound and outbound traffic immediately severs the instance's network path at the hypervisor, cutting off the C2 server connection without killing the process or losing memory artifacts. Because security group changes are applied instantly across the VPC, this containment step is faster than OS-level firewall rules and avoids tipping off the attacker. The instance remains powered on, allowing you to capture a memory dump and disk image for forensic analysis before any restoration or termination.
Why this answer
Applying a new security group that denies all inbound and outbound traffic immediately stops the C2 communication at the network layer without destroying the instance. This preserves the instance for forensic analysis (e.g., memory dump, disk imaging) while containing the threat. The security group acts as a virtual firewall, and changing it is a non-destructive, reversible action that can be applied directly to the instance even if it is part of an Auto Scaling group.
Exam trap
The trap here is that candidates often choose to terminate the instance (Option C) thinking it is the fastest containment, but they overlook the critical requirement to preserve the instance for forensic analysis, which termination destroys.
How to eliminate wrong answers
Option B is wrong because removing the security group from the Auto Scaling group does not isolate the instance; the instance retains its existing security group(s) and continues to communicate. Option C is wrong because terminating the instance destroys the forensic evidence (e.g., volatile memory, running processes, disk state) and prevents further analysis. Option D is wrong because creating an AMI takes time and does not immediately stop the C2 communication; the instance remains active and can continue exfiltrating data or receiving commands during the AMI creation process.