The answer is to disable VRFY and EXPN commands in the SMTP server configuration, as this directly stops user enumeration during the reconnaissance phase. VRFY, defined in RFC 821, lets an attacker verify if an email address exists, while EXPN reveals mailing list members; both are abused to map valid accounts without authentication. On the Certified Ethical Hacker CEH exam, this tests your understanding of SMTP reconnaissance techniques and the principle of reducing the attack surface by disabling unnecessary protocol features. A common trap is to suggest rate-limiting or logging instead, but those only detect or slow the attack, not prevent it—disabling the commands is the definitive mitigation. Remember the mnemonic: “VRFY and EXPN are the attacker’s pen; disable them to end the recon again.”
CEH Enumeration and System Hacking Practice Question
This CEH practice question tests your understanding of enumeration and system hacking. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A security analyst observes repeated attempts to validate user accounts via SMTP using VRFY commands from an external IP. What is the BEST immediate action to mitigate this reconnaissance?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Disable VRFY and EXPN commands in the SMTP server configuration
The VRFY command is an SMTP protocol feature (RFC 821) that allows a client to verify whether a specific email address exists on the server. Attackers use VRFY and EXPN to enumerate valid user accounts during the reconnaissance phase. Disabling these commands in the SMTP server configuration directly stops this enumeration technique without affecting legitimate mail delivery.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Upgrade to the latest version of the SMTP server
Why it's wrong here
Upgrading may not remove the commands.
✗
Enable SMTP authentication for all incoming connections
Why it's wrong here
Authentication does not stop VRFY if allowed.
✗
Block the external IP address at the firewall
Why it's wrong here
IP blocking is reactive; the attacker can use different IPs.
✓
Disable VRFY and EXPN commands in the SMTP server configuration
Why this is correct
This prevents the enumeration technique entirely.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often choose to block the IP address (Option C) as a quick fix, failing to recognize that the root cause is the enabled VRFY/EXPN commands, which must be disabled at the server configuration level for a permanent mitigation.
Trap categories for this question
Command / output trap
Upgrading may not remove the commands.
Detailed technical explanation
How to think about this question
SMTP VRFY and EXPN commands are part of the original SMTP specification and are often enabled by default in legacy mail servers. Even with modern SMTP extensions like STARTTLS, these commands can be issued before encryption is negotiated. In a real-world scenario, an attacker might combine VRFY with dictionary-based username guesses to build a valid user list for subsequent phishing or brute-force attacks.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the CEH exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CEH question in full detail.
Enumeration and System Hacking — This question tests Enumeration and System Hacking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Disable VRFY and EXPN commands in the SMTP server configuration — The VRFY command is an SMTP protocol feature (RFC 821) that allows a client to verify whether a specific email address exists on the server. Attackers use VRFY and EXPN to enumerate valid user accounts during the reconnaissance phase. Disabling these commands in the SMTP server configuration directly stops this enumeration technique without affecting legitimate mail delivery.
What should I do if I get this CEH question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. An attacker uses SMTP commands to verify the existence of email accounts on a mail server. Which sequence of SMTP commands is used for this purpose?
hard
A.EHLO, AUTH, STARTTLS
B.HELO, MAIL FROM, RCPT TO, DATA
C.NOOP, QUIT, RSET
✓ D.VRFY, EXPN, RCPT TO
Why D: Option D is correct because the VRFY command asks the mail server to verify whether a given email address exists, EXPN expands a mailing list to reveal individual addresses, and RCPT TO (used in the SMTP transaction) can also be used to check address validity by observing the server's response. Together, these commands allow an attacker to enumerate valid email accounts on the server.
Variation 2. A penetration tester is attempting to enumerate user accounts on a mail server. They connect to port 25 and issue the commands `VRFY root` and `EXPN support`. Which protocol is being targeted?
medium
A.LDAP
✓ B.SMTP
C.SMB
D.SNMP
Why B: The commands VRFY and EXPN are SMTP (Simple Mail Transfer Protocol) commands defined in RFC 821 and RFC 5321. VRFY is used to verify whether a mailbox exists on the server, while EXPN expands a mailing list alias to reveal its members. Since the tester connected to port 25 (the default SMTP port) and issued these commands, SMTP is the protocol being targeted.
Variation 3. A penetration tester wants to enumerate user accounts on a Linux system running SMTP service. Which commands are commonly used for this purpose?
medium
A.USER, PASS, QUIT
✓ B.VRFY, EXPN, RCPT TO
C.AUTH LOGIN, STARTTLS, EHLO
D.HELO, MAIL FROM, DATA
Why B: The VRFY command verifies whether a user mailbox exists on the SMTP server, EXPN expands a mailing list or alias to reveal individual member addresses, and RCPT TO specifies the recipient for a mail message and can be used to validate addresses during the SMTP conversation. These three commands are the standard SMTP enumeration techniques for discovering valid user accounts on a Linux system running an SMTP service.
Last reviewed: Jun 24, 2026
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This CEH practice question is part of Courseiva's free EC-Council 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 CEH exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.