CS0-003 Security Operations Practice Question
A security analyst is investigating a series of alerts from AWS GuardDuty indicating 'UnauthorizedAccess:EC2/SSHBruteForce'. The affected EC2 instance has a high CPU load. The analyst checks the security group rules and finds that SSH (port 22) is open to 0.0.0.0/0. What is the best immediate remediation action?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a new security group allowing SSH only from known IPs and attach it to the instance
The instance is exposed to the internet on SSH. Restricting SSH access to known IP addresses reduces the attack surface. Creating a new security group with a narrow source range and applying it stops brute force attempts.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Create a new security group allowing SSH only from known IPs and attach it to the instance
Why this is correct
Modifying the AWS security group to restrict SSH access to specific, trusted source IP addresses immediately mitigates the brute-force or unauthorized access attempt. This stateful firewall change is applied at the hypervisor level, ensuring legitimate administrative access is preserved while blocking malicious traffic before it reaches the operating system.
- ✗
Terminate the EC2 instance immediately
Why it's wrong here
Immediately terminating the EC2 instance is an overreaction that disrupts business operations and destroys volatile forensic evidence, such as memory contents and active network connections. Containment should prioritize isolating the threat via network controls rather than destroying the asset before a proper investigation can occur.
- ✗
Disable the GuardDuty alert to reduce noise
Why it's wrong here
Silencing or disabling Amazon GuardDuty alerts merely hides the symptoms of a potential compromise without addressing the underlying vulnerability. This action leaves the instance exposed to ongoing attacks and severely degrades the organization's security visibility, violating basic incident response principles.
- ✗
Install a host-based firewall on the instance to block SSH
Why it's wrong here
Attempting to install and configure a host-based firewall on the instance during an active alert is inefficient and risks completely locking out authorized administrators. Furthermore, blocking all SSH traffic at the host level prevents necessary remote management, whereas security groups can filter traffic externally without OS-level intervention.
Go deeper
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DevSecOps and Shifting Security Left
Key term
Security
Security in IT is the practice of protecting systems, networks, and data from unauthorized access, damage, or theft.
Key term
GuardDuty
AWS GuardDuty is a managed threat detection service that continuously monitors for malicious activity and unauthorized behavior in an AWS environment.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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