The answer is SMTP user enumeration via VRFY. This is correct because the VRFY command is designed to verify whether a specific user mailbox exists on a mail server, and the varying response codes—250 for a confirmed user, 252 for an ambiguous result, and 550 for an unknown user—allow an attacker to systematically map valid usernames. On the Certified Ethical Hacker CEH exam, this technique tests your understanding of reconnaissance methods that exploit SMTP protocol commands, often appearing in questions about footprinting and enumeration. A common trap is confusing VRFY with EXPN (which expands mailing lists) or assuming a 252 response means the user is invalid; in reality, 252 indicates the server cannot confirm but will still accept mail for that address. For a quick memory tip, remember the three response codes as “250 = yes, 252 = maybe, 550 = no,” which directly maps to user existence during SMTP VRFY user enumeration.
CEH Enumeration and System Hacking Practice Question
This CEH practice question tests your understanding of enumeration and system hacking. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 captures the following SMTP conversation: 220 mail.example.com ESMTP; HELO client; 250 Hello; VRFY root; 250 Super-User; VRFY admin; 252 Cannot VRFY user; VRFY user1; 550 User unknown. Which attack is the analyst performing?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
SMTP user enumeration via VRFY
Option C is correct. The VRFY command is used to verify email addresses. A successful response (250) indicates the user exists; 252 indicates ambiguous; 550 indicates no such user. This is enumeration of valid usernames.
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.
✗
SMTP header injection
Why it's wrong here
Header injection involves inserting additional headers into email messages.
✗
SMTP open relay testing
Why it's wrong here
Open relay testing involves sending email through the server without authentication.
✓
SMTP user enumeration via VRFY
Why this is correct
VRFY queries verify if a mailbox exists; responses reveal valid usernames.
Related concept
Read the scenario before looking for a memorised answer.
✗
SMTP DDoS attack
Why it's wrong here
DDoS would involve overwhelming the server with requests, not a few VRFY commands.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Trap categories for this question
Command / output trap
DDoS would involve overwhelming the server with requests, not a few VRFY commands.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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.
Identify which CEH exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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: SMTP user enumeration via VRFY — Option C is correct. The VRFY command is used to verify email addresses. A successful response (250) indicates the user exists; 252 indicates ambiguous; 550 indicates no such user. This is enumeration of valid usernames.
What should I do if I get this CEH question wrong?
Identify which CEH exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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