AWS Certified Security – Specialty (SCS-C02) — Question 306

A company's security engineer is designing an isolation procedure for Amazon EC2 instances as part of an incident response plan. The security engineer needs to isolate a target instance to block any traffic to and from the target instance, except for traffic from the company's forensics team. Each of the company's EC2 instances has its own dedicated security group. The EC2 instances are deployed in subnets of a VPC. A subnet can contain multiple instances.
The security engineer is testing the procedure for EC2 isolation and opens an SSH session to the target instance. The procedure starts to simulate access to the target instance by an attacker. The security engineer removes the existing security group rules and adds security group rules to give the forensics team access to the target instance on port 22.
After these changes, the security engineer notices that the SSH connection is still active and usable. When the security engineer runs a ping command to the public IP address of the target instance, the ping command is blocked.
What should the security engineer do to isolate the target instance?

Answer options

• A. Add an inbound rule to the security group to allow traffic from 0.0.0.0/0 for all ports. Add an outbound rule to the security group to allow traffic to 0.0.0.0/0 for all ports. Then immediately delete these rules.
• B. Remove the port 22 security group rule. Attach an instance role policy that allows AWS Systems Manager Session Manager connections so that the forensics team can access the target instance.
• C. Create a network ACL that is associated with the target instance's subnet. Add a rule at the top of the inbound rule set to deny all traffic from 0.0.0.0/0. Add a rule at the top of the outbound rule set to deny all traffic to 0.0.0.0/0.
• D. Create an AWS Systems Manager document that adds a host-level firewall rule to block all inbound traffic and outbound traffic. Run the document on the target instance.

Correct answer: B

Explanation

Option B is correct because removing the port 22 security group rule entirely prevents any standard SSH connections, terminating existing sessions and blocking new ones. Using AWS Systems Manager Session Manager allows the forensics team to securely access the instance via IAM policies without requiring any open inbound ports. Option C is incorrect because applying a deny-all Network ACL at the subnet level would affect all other instances in that subnet and block the forensics team's access as well.