The answer is implementing a web application firewall (WAF) as the best compensating control for an unpatched vulnerable web server. A WAF operates at Layer 7 to inspect HTTP/HTTPS traffic and can block known attack patterns, such as remote code execution payloads targeting specific CVEs like CVE-2021-41773, effectively providing virtual patching without altering the server itself. On the CISSP exam, this scenario tests your understanding of compensating controls within the risk management framework—specifically how to reduce risk when a primary control (patching) is temporarily infeasible. A common trap is choosing network segmentation or host-based intrusion prevention, but those lack the application-layer granularity needed to block web-specific exploits. Remember the mnemonic “WAF for Web” to recall that application-layer threats require application-layer defenses.
CISSP Security Operations Practice Question
This CISSP practice question tests your understanding of security operations. 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.
During a vulnerability scan, a security analyst discovers that a web server is running an outdated version of Apache with known remote code execution vulnerabilities. The server is in production and cannot be patched immediately due to dependency conflicts. What is the best compensating control to reduce risk while a permanent fix is developed?
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.
Clue: "immediately / without restart"
Why it matters: Time or reboot constraint — the correct answer must take effect right away without requiring a reboot or reload.
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) to block known attack patterns
A Web Application Firewall (WAF) is the best compensating control because it can inspect HTTP/HTTPS traffic at the application layer (Layer 7) and block known attack patterns targeting the outdated Apache version, such as specific payloads for CVE-2017-9798 or CVE-2021-41773. Unlike other options, a WAF provides virtual patching without modifying the vulnerable server, directly mitigating the remote code execution risk while a permanent fix is developed.
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.
✗
Disable all unnecessary modules and services on the server
Why it's wrong here
Reducing attack surface is good practice but does not address the known vulnerability.
✓
Implement a web application firewall (WAF) to block known attack patterns
Why this is correct
A WAF can detect and block exploit attempts, providing virtual patching until the software is updated.
Clue confirmation
The clue words "best", "immediately / without restart" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Enable detailed logging and alerting for the server
Why it's wrong here
Logging helps detect attacks but does not prevent them.
✗
Isolate the server in a separate VLAN with strict ACLs
Why it's wrong here
Segmentation helps but does not prevent exploitation of the vulnerability if the server is still accessible.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the distinction between detective controls (logging/alerting) and preventive controls (WAF), leading candidates to choose logging because it seems proactive, but it fails to reduce risk in real time.
Detailed technical explanation
How to think about this question
A WAF operates by analyzing HTTP request headers, parameters, and payloads against a rule set (e.g., OWASP ModSecurity Core Rule Set) to detect and block exploit attempts like path traversal or buffer overflow patterns. In a real-world scenario, a WAF can be configured in reverse proxy mode to terminate TLS and inspect decrypted traffic, ensuring encrypted attacks are also blocked. This approach is often called 'virtual patching' and is widely used in PCI DSS environments to buy time for patch deployment.
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 security team runs a vulnerability scan on a web application and discovers an unpatched SQL injection flaw. The team prioritises remediation by CVSS score — critical flaws are patched within 24 hours, high within 7 days. Questions like this test whether you understand vulnerability management processes, scanning tools, and remediation prioritisation.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Security Operations — This question tests Security Operations — 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) to block known attack patterns — A Web Application Firewall (WAF) is the best compensating control because it can inspect HTTP/HTTPS traffic at the application layer (Layer 7) and block known attack patterns targeting the outdated Apache version, such as specific payloads for CVE-2017-9798 or CVE-2021-41773. Unlike other options, a WAF provides virtual patching without modifying the vulnerable server, directly mitigating the remote code execution risk while a permanent fix is developed.
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", "immediately / without restart". 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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