CAS-004 Security Engineering Practice Question
During a security audit, it is discovered that a critical server uses SSH with password authentication and supports weak key exchange algorithms. Which of the following is the most effective hardening step to prevent brute-force attacks and ensure forward secrecy?
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
✓
Disable password authentication and restrict key exchange algorithms to curve25519-sha256
Disabling password authentication forces the use of key-based authentication, which is resistant to brute-force attacks. Restricting key exchange algorithms to those providing forward secrecy (e.g., Curve25519) ensures that session keys cannot be compromised even if the long-term private key is exposed.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Implement fail2ban to block IPs after failed attempts
Why it's wrong here
Fail2ban helps but does not eliminate the risk of password guessing.
- ✓
Disable password authentication and restrict key exchange algorithms to curve25519-sha256
Why this is correct
This eliminates password vulnerabilities and ensures forward secrecy.
- ✗
Change the SSH port to a non-default high port
Why it's wrong here
Security through obscurity; does not prevent brute force if password auth remains.
- ✗
Use RSA keys with 4096-bit length
Why it's wrong here
While strong, RSA does not provide forward secrecy by itself; key exchange algorithms should be selected.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CAS-005 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CAS-005 exam.