The answer is null byte injection, which is the correct choice because the attacker exploits the %00 (null byte) character to terminate the filename string prematurely, causing the server to ignore the appended '.png' extension and save the file as 'malicious.php'. This bypasses the application’s extension validation, allowing arbitrary PHP code to be uploaded and executed. On the Certified Ethical Hacker CEH exam, this vulnerability tests your understanding of file upload attack vectors and input sanitization flaws, often appearing in web application security scenarios where the server relies on user-supplied filenames without proper filtering. A common trap is confusing this with MIME type or content-type manipulation, but the key is the null byte’s string-terminating behavior in languages like C or PHP. Memory tip: think of the null byte as a “string cutter”—%00 snips off everything after it, just like a pair of scissors trimming a filename.
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 allows users to upload profile pictures. The application uses the filename provided by the user to save the file on the server. An attacker uploads a file named 'malicious.php%00.png' and the server saves it as 'malicious.php'. Which vulnerability is being exploited?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Null byte injection
This is a null byte injection attack, where the %00 (null byte) terminates the string, bypassing extension checks to upload a PHP file.
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.
✗
Directory traversal
Why it's wrong here
Directory traversal involves path manipulation to access restricted directories; not related to null byte extension bypass.
✗
Command injection
Why it's wrong here
Command injection involves executing system commands via input; not related to file upload extension manipulation.
✓
Null byte injection
Why this is correct
Null byte injection exploits the null byte character to truncate strings, bypassing file extension filters.
Related concept
Static NAT maps one inside address to one outside address.
✗
Cross-Site Scripting (XSS)
Why it's wrong here
XSS involves injecting scripts into web pages; not related to file upload extension manipulation.
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.
Trap categories for this question
Command / output trap
Command injection involves executing system commands via input; not related to file upload extension manipulation.
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: Null byte injection — This is a null byte injection attack, where the %00 (null byte) terminates the string, bypassing extension checks to upload a PHP file.
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.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Question Discussion
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