The answer is using inline comments like 'UN/**/ION/**/SE/**/LECT' to bypass the WAF. This technique works because the web application firewall is looking for the exact string 'UNION SELECT' as a contiguous keyword sequence, but SQL parsers ignore inline comments between keywords, effectively reconstructing the original command during execution. On the Certified Ethical Hacker CEH exam, this tests your understanding of WAF evasion tactics within the SQL injection domain, often appearing in scenario-based questions where a signature-based filter blocks common SQL patterns. A common trap is assuming URL encoding or case variation will work, but most modern WAFs normalize those; inline comment injection directly exploits the gap between WAF pattern matching and SQL interpreter logic. Memory tip: think of it as "breaking the keyword chain" — insert a comment block between each syllable of the forbidden word, and the database will reassemble the command while the WAF sees only fragments.
CEH Web Application and Injection Attacks Practice Question
This CEH practice question tests your understanding of web application and injection attacks. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 web application firewall (WAF) blocks requests containing ' UNION SELECT '. A penetration tester wants to bypass this restriction to perform a union-based SQL injection. Which of the following techniques is MOST likely to succeed?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use inline comments: 'UN/**/ION/**/SE/**/LECT'
Using comments or alternative encoding can bypass WAF rules. Inline comments like '/**/' can break up keywords.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Use double URL encoding: '%25%35%35%25%34%65%25%34%39...'
Why it's wrong here
Double encoding might bypass some WAFs but is less reliable than comments.
✗
Use hex encoding: '0x554e494f4e2053454c454354'
Why it's wrong here
Hex encoding is not directly usable in SQL injection without proper context.
✗
Use URL encoding: '%55%4e%49%4f%4e%20%53%45%4c%45%43%54'
Why it's wrong here
WAFs typically decode URL encoding before inspection.
✓
Use inline comments: 'UN/**/ION/**/SE/**/LECT'
Why this is correct
Inline comments break up keywords and evade simple signature-based WAF rules.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CEH question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related CEH NAT questions on configuration and troubleshooting.
Web Application and Injection Attacks — This question tests Web Application and Injection Attacks — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Use inline comments: 'UN/**/ION/**/SE/**/LECT' — Using comments or alternative encoding can bypass WAF rules. Inline comments like '/**/' can break up keywords.
What should I do if I get this CEH question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related CEH NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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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. A security team deployed a web application firewall (WAF) that blocks requests containing SQL keywords like 'SELECT', 'UNION', and 'DROP'. An attacker bypasses the WAF by encoding the payload in base64 and using a SQL injection tool that decodes it server-side. Which mitigation would be MOST effective against this?
medium
A.Increase WAF sensitivity to block base64-encoded strings
✓ B.Implement prepared statements with parameterized queries
C.Disable error messages to hide database errors
D.Use a more restrictive input validation regex
Why B: Prepared statements (parameterized queries) prevent SQL injection by separating query structure from data, regardless of encoding. WAF bypasses are mitigated by using proper coding practices.
Last reviewed: Jun 21, 2026
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