The correct answer is implementing an allowlist of permitted URLs or IP addresses and disabling unused URL schemas, as these two techniques directly address the core vulnerability in Server-Side Request Forgery (SSRF). An allowlist restricts the application to only make requests to trusted destinations, preventing arbitrary internal or external targets from being accessed, while disabling unused URL schemas—such as file://, dict://, or gopher://—eliminates alternative attack vectors that bypass standard HTTP validation. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of defense-in-depth for web application attacks, often appearing in scenario-based questions where input validation is presented as a distractor because it fails to block protocol-level abuse. A common trap is assuming that blacklisting dangerous IP ranges is sufficient, but attackers can easily bypass such lists with DNS rebinding or URL obfuscation. Remember the memory tip: “Allow what you trust, disable what you don’t use”—this dual approach ensures both destination control and protocol restriction.
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 security analyst identifies that a web application is vulnerable to Server-Side Request Forgery (SSRF). Which TWO of the following are effective mitigation techniques for SSRF?
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
Disabling unused schemas limits the protocols an attacker can abuse.
Related concept
Standard ACLs match source addresses.
✗
Use a blacklist to block private IP ranges
Why it's wrong here
Blacklists can be bypassed with DNS rebinding or alternative representations.
✗
Increase the timeout for HTTP requests
Why it's wrong here
Increasing timeout does not mitigate the vulnerability.
✓
Implement an allowlist of permitted URLs or IP addresses
Why this is correct
An allowlist restricts the server to making requests only to trusted destinations.
Related concept
Standard ACLs match source addresses.
✗
Encode user input in base64 before passing to URL functions
Why it's wrong here
Encoding does not prevent SSRF; it can be decoded by the server.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CEH question in full detail.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CEH ACL questions on filtering logic and placement.
Web Application and Injection Attacks — This question tests Web Application and Injection Attacks — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Disable unnecessary URL schemas (e.g., file://, dict://) — Whitelisting allowed domains prevents requests to arbitrary targets. Disabling unused URL schemas (e.g., file://) reduces attack surface. Input validation alone is insufficient.
What should I do if I get this CEH question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CEH ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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Question Discussion
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