- A
Using HTTPS to encrypt traffic
Why wrong: HTTPS protects data in transit but does not prevent XSS.
- B
Implementing a Content Security Policy (CSP) with strict directives
Why wrong: CSP is a defense-in-depth layer but not the most effective; output encoding directly prevents injection.
- C
Validating input against a whitelist of allowed characters
Why wrong: Whitelist validation can reduce risks but may not cover all contexts; output encoding is primary.
- D
Encoding all user-supplied data before reflecting it in the response
Output encoding (e.g., HTML encoding) is the primary defense against reflected XSS.
Question 998 of 1,000
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 security team is conducting a penetration test on a web application. They identify that the application is vulnerable to reflected cross-site scripting (XSS). Which of the following is the most effective mitigation?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Encoding all user-supplied data before reflecting it in the response
Option D is correct because reflecting user-supplied data without proper encoding allows an attacker to inject arbitrary HTML/JavaScript that executes in the victim's browser. Output encoding (e.g., HTML entity encoding for context like <script> to <script>) neutralizes the injected script by treating it as data rather than executable code. This directly addresses the root cause of reflected XSS—failure to separate user input from executable content in the response.
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.
- ✗
Using HTTPS to encrypt traffic
Why it's wrong here
HTTPS protects data in transit but does not prevent XSS.
- ✗
Implementing a Content Security Policy (CSP) with strict directives
Why it's wrong here
CSP is a defense-in-depth layer but not the most effective; output encoding directly prevents injection.
- ✗
Validating input against a whitelist of allowed characters
Why it's wrong here
Whitelist validation can reduce risks but may not cover all contexts; output encoding is primary.
- ✓
Encoding all user-supplied data before reflecting it in the response
Why this is correct
Output encoding (e.g., HTML encoding) is the primary defense against reflected XSS.
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 confuse input validation (Option C) with output encoding, but the CISSP emphasizes that output encoding is the definitive control for injection flaws because it ensures data is treated as data regardless of input validation failures.
Trap categories for this question
Command / output trap
CSP is a defense-in-depth layer but not the most effective; output encoding directly prevents injection.
Detailed technical explanation
How to think about this question
Reflected XSS occurs when an application immediately includes unvalidated and unencoded user input in an HTTP response without proper contextual output encoding (e.g., HTML, JavaScript, CSS, URL contexts). The OWASP XSS Prevention Cheat Sheet specifies that output encoding must be context-aware—for example, using `htmlspecialchars()` in PHP with ENT_QUOTES for HTML body context, or JavaScript hex escaping for event handler attributes. In real-world scenarios, even a single missed encoding point (e.g., in a JSON callback or a custom header) can lead to exploitation, as seen in the 2018 British Airways breach where a reflected XSS in a payment form was used to steal credit card data.
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 SOC analyst notices unusual lateral movement in the network at 2 AM. The IR playbook dictates: identify and contain (isolate the affected machine), then eradicate (remove the malware), then recover (restore from backup), then document. Skipping containment before eradication risks the attacker regaining access. Questions like this test the sequence and rationale of incident response phases.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this CISSP question test?
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: Encoding all user-supplied data before reflecting it in the response — Option D is correct because reflecting user-supplied data without proper encoding allows an attacker to inject arbitrary HTML/JavaScript that executes in the victim's browser. Output encoding (e.g., HTML entity encoding for context like <script> to <script>) neutralizes the injected script by treating it as data rather than executable code. This directly addresses the root cause of reflected XSS—failure to separate user input from executable content in the response.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
This CISSP practice question is part of Courseiva's free ISC2 certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CISSP exam.
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