The answer is to implement a Web Application Firewall (WAF) rule to block exploitation, as this provides an immediate compensating control that mitigates risk from the vulnerable third-party library without requiring months of code rewrites. This is correct because a WAF operates at the application layer, inspecting HTTP/HTTPS traffic for attack patterns like SQL injection or path traversal specific to the library, effectively buying time for a permanent fix while aligning with the defense-in-depth principle. On the CISSP exam, this scenario tests your ability to prioritize risk reduction over remediation speed, often appearing in questions about software supply chain security or vulnerability management. A common trap is choosing to patch or rewrite immediately, which is impractical here; instead, remember that compensating controls like WAF rules are the best immediate action when a direct fix is delayed. Memory tip: “WAF buys time while devs climb the climb.”
CISSP Software Development Security Practice Question
This CISSP practice question tests your understanding of software development security. 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 software company uses a third-party library that has a known critical vulnerability. The library is used extensively and rewriting the code would take months. What is the BEST immediate action to reduce risk?
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
✓
Implement a Web Application Firewall (WAF) rule to block exploitation
Option D is correct because implementing a Web Application Firewall (WAF) rule to block exploitation provides an immediate, compensating control that mitigates the known vulnerability without requiring code changes. This is the best immediate action because it buys time for a permanent fix while reducing risk, aligning with the principle of defense in depth. The WAF can inspect HTTP/HTTPS traffic for attack patterns (e.g., SQL injection, path traversal) specific to the vulnerable library and block malicious requests at the application layer.
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.
✗
Remove the library from the codebase immediately
Why it's wrong here
This would break functionality and is not feasible without extensive rewriting.
✗
Disable the vulnerable feature in the library
Why it's wrong here
Disabling a feature may not be possible without modifying the library's source code.
✗
Increase logging and monitoring to detect exploitation attempts
Why it's wrong here
This only detects, not prevents, exploitation; it is a detective control, not a preventive one.
✓
Implement a Web Application Firewall (WAF) rule to block exploitation
Why this is correct
A WAF can provide virtual patching to mitigate the vulnerability in transit.
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 'Remove the library immediately' (Option A) because it seems like the most direct fix, but they fail to consider the business continuity impact and the need for a risk-based, phased approach to remediation.
Detailed technical explanation
How to think about this question
A WAF operates at Layer 7 (application layer) and can use positive security models (allowlisting) or negative security models (signature-based blocking) to filter malicious payloads. For example, if the vulnerable library is used for XML parsing, a WAF rule can block XXE (XML External Entity) attacks by inspecting Content-Type headers and request bodies for DOCTYPE declarations. In a real-world scenario, the Equifax breach (2017) exploited a known vulnerability in Apache Struts; a WAF rule could have blocked the specific OGNL injection payloads, potentially preventing the breach while the patch was being applied.
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 developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Software Development Security — This question tests Software Development Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Implement a Web Application Firewall (WAF) rule to block exploitation — Option D is correct because implementing a Web Application Firewall (WAF) rule to block exploitation provides an immediate, compensating control that mitigates the known vulnerability without requiring code changes. This is the best immediate action because it buys time for a permanent fix while reducing risk, aligning with the principle of defense in depth. The WAF can inspect HTTP/HTTPS traffic for attack patterns (e.g., SQL injection, path traversal) specific to the vulnerable library and block malicious requests at the application layer.
What should I do if I get this CISSP 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.
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Question Discussion
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