XK0-006 Security Practice Question
A Linux engineer needs to harden SSH access. Which TWO of the following settings should be configured in /etc/ssh/sshd_config to enhance security? (Select TWO.)
⚠ Common exam trap
Watch out — candidates often confuse 'hardening' with 'increasing limits' (like MaxAuthTries) or 'enabling convenience' (like PermitRootLogin yes), when the correct hardening choices actually restrict or disable weaker authentication methods.
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
✓
PasswordAuthentication no
Option B (PasswordAuthentication no) is correct because disabling password-based authentication forces users to authenticate with SSH key pairs, eliminating brute-force and credential-stuffing attacks against account passwords. Option E (AllowUsers alice bob) is correct because it implements an explicit allowlist, so only the named accounts alice and bob may establish SSH sessions, denying all other valid system users. The remaining options weaken security: MaxAuthTries 6 (A) is too permissive since the default of 3 limits failed attempts more tightly, Protocol 1 (C) enables the obsolete and cryptographically broken SSH-1 protocol, and PermitRootLogin yes (D) allows direct root logins, which should be set to no or prohibit-password.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
MaxAuthTries 6
Why it's wrong here
MaxAuthTries 6 permits six authentication attempts per connection, giving brute-force attempts more room than a hardened value such as 3 or 4. It is tempting because it is a genuine sshd_config authentication directive, and limiting attempts at all feels like hardening, but the threshold is too permissive.
- ✓
PasswordAuthentication no
Why this is correct
Disables password logins, reducing risk of brute force.
- ✗
Protocol 1
Why it's wrong here
Protocol 1 is the obsolete SSH-1 protocol, which has known cryptographic weaknesses and is disabled in modern OpenSSH builds; hardening requires Protocol 2 only. It is tempting because it appears to specify an SSH protocol version explicitly, and older documentation listed it as a configurable setting.
- ✗
PermitRootLogin yes
Why it's wrong here
PermitRootLogin yes allows direct root logins over SSH, removing the accountability and privilege separation that sudo-based administration provides. It is tempting because it simplifies administrative access when engineers want unrestricted root sessions, but hardening requires disabling or restricting it, typically to prohibit-password.
- ✓
AllowUsers alice bob
Why this is correct
AllowUsers defines an explicit allowlist, so only alice and bob may authenticate over SSH; every other account is refused regardless of valid credentials. This meets the hardening requirement by shrinking the set of accounts exposed to remote login attempts.
Go deeper
Related to this question
Learn chapter
Installing Linux and Package Management
Key term
Linux
Linux is an open-source operating system that manages computer hardware and software, widely used in servers, desktops, and embedded systems.
Key term
SSH
SSH (Secure Shell) is a cryptographic network protocol that provides secure, encrypted communication and remote administration between two devices over an unsecured network.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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