This CCSP practice question tests your understanding of cloud application security. 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.
A software company develops a mobile application that communicates with a cloud backend using REST APIs. The application uses OAuth 2.0 with the authorization code grant and PKCE for authentication. After a security audit, the team identifies that the backend API accepts both a client secret (from the authorization code grant) and a PKCE code verifier. The security team wants to remove unnecessary attack surface. Which change should be made?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Remove the client_secret parameter from the token endpoint and rely solely on PKCE
Option C is correct because PKCE (Proof Key for Code Exchange, RFC 7636) was specifically designed to secure the authorization code grant for public clients like mobile apps, where a client secret cannot be reliably kept confidential. By removing the client_secret parameter and relying solely on PKCE, the team eliminates an unnecessary attack surface—since the secret is effectively a static credential that can be extracted from the app binary—while maintaining strong protection against authorization code interception attacks. The backend should enforce PKCE verification using the code_challenge and code_verifier, making the client_secret redundant for public clients.
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.
✗
Switch to the implicit grant (response_type=token) to avoid client secrets
Why it's wrong here
Implicit grant is less secure and deprecated; PKCE is the modern standard.
✗
Keep both mechanisms but use short-lived tokens to reduce risk
Why it's wrong here
Both mechanisms add complexity and risk; PKCE alone suffices.
✓
Remove the client_secret parameter from the token endpoint and rely solely on PKCE
Why this is correct
PKCE is designed for public clients without a secret, reducing attack surface.
Related concept
Read the scenario before looking for a memorised answer.
✗
Require a stronger client secret (e.g., 256-bit) and store it in the app's encrypted storage
Why it's wrong here
Encrypted storage on mobile can still be bypassed; secrets are inherently compromised.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the misconception that removing the client_secret weakens security, when in fact for public clients (mobile apps, SPAs) PKCE makes the secret unnecessary and its removal reduces attack surface; candidates may incorrectly think keeping the secret adds a layer of defense, but it actually introduces a static credential that can be stolen.
Detailed technical explanation
How to think about this question
Under the hood, PKCE works by having the client generate a cryptographically random code_verifier (a high-entropy string, typically 43–128 characters) and send its SHA-256 hash (the code_challenge) during the authorization request. During the token exchange, the client sends the original code_verifier, and the server verifies it matches the stored challenge—this ensures that even if the authorization code is intercepted, the attacker cannot exchange it without the verifier. A real-world scenario is a mobile banking app: if the client_secret is embedded and an attacker reverse-engineers the app, they can impersonate the client at the token endpoint; with PKCE alone, the secret is removed, and the verifier is ephemeral and never stored on the device long-term.
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.
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.
Cloud Application Security — This question tests Cloud Application Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Remove the client_secret parameter from the token endpoint and rely solely on PKCE — Option C is correct because PKCE (Proof Key for Code Exchange, RFC 7636) was specifically designed to secure the authorization code grant for public clients like mobile apps, where a client secret cannot be reliably kept confidential. By removing the client_secret parameter and relying solely on PKCE, the team eliminates an unnecessary attack surface—since the secret is effectively a static credential that can be extracted from the app binary—while maintaining strong protection against authorization code interception attacks. The backend should enforce PKCE verification using the code_challenge and code_verifier, making the client_secret redundant for public clients.
What should I do if I get this CCSP 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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Question Discussion
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