Authorization Code Flow with PKCE — Single-Page App Authentication
A single-page app signs in users with Microsoft Entra ID and calls a protected API. The app cannot safely keep a client secret. Which OAuth flow should be used?
Quick Answer
The answer is the authorization code flow with PKCE. This is the correct choice because a single-page app (SPA) running in a browser cannot securely store a client secret, making the standard authorization code flow vulnerable to interception. PKCE (Proof Key for Code Exchange) mitigates this by requiring the app to generate a cryptographic code verifier and its transformed code challenge during the initial request; when the authorization code is later exchanged for tokens, the server verifies that the presenter possesses the original verifier, blocking code injection attacks even if the code is intercepted. On the AZ-204 exam, this scenario tests your understanding of secure authentication patterns for browser-based apps, often appearing as a trap where candidates mistakenly choose the implicit grant (now deprecated) or the client credentials flow. Remember the key distinction: PKCE uses a dynamically generated secret per request, not a static client secret. Memory tip: think “PKCE = Proof Key = Prove you have the key, no secret needed.”
⚠ Common exam trap
A common mix-up: candidates confuse the implicit flow (which was historically used for SPAs) as still valid, but Microsoft and OAuth standards now mandate the authorization code flow with PKCE for all public clients, including single-page apps.
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
✓
Authorization code flow with PKCE
The authorization code flow with PKCE (Proof Key for Code Exchange) is the recommended OAuth flow for single-page apps that cannot securely store a client secret. PKCE ensures that even if the authorization code is intercepted, it cannot be exchanged for tokens without the original code verifier, mitigating the risk of code injection attacks. This flow aligns with Microsoft's best practices for native and browser-based applications using Microsoft Entra ID.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Implicit flow
Why it's wrong here
Implicit flow returns tokens directly from the authorisation endpoint without a code exchange, and Microsoft now discourages it for SPAs because it cannot use PKCE or refresh tokens. It was designed for browser-based apps before authorisation code with PKCE became viable.
- ✗
Client credentials flow
Why it's wrong here
Client credentials authenticates the application itself, with no user context, so it cannot sign in users or issue delegated tokens for a protected API. It is tempting because it suits daemon or service-to-service calls where no interactive user exists and the app can hold a secret.
- ✗
Resource owner password credentials flow
Why it's wrong here
Resource owner password credentials flow requires the app to handle the user's password directly and offers no consent or modern MFA support, so it cannot secure a browser-based SPA. It is tempting for legacy or trusted first-party clients, where interactive redirect flows are impossible.
- ✓
Authorization code flow with PKCE
Why this is correct
Authorization code flow with PKCE uses a dynamically generated code verifier and challenge, so the public single-page app proves possession without storing a client secret. This satisfies the constraint that the app cannot safely keep a client secret while calling a protected API.
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Same concept, more angles
3 more ways this is tested on AZ-204
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A single-page app signs in users with Microsoft Entra ID and calls a protected API. The app cannot safely keep a client secret. Which OAuth flow should be used? The design must avoid adding custom operational scripts.
hard- A.Implicit flow
- B.Client credentials flow
- C.Resource owner password credentials flow
- ✓ D.Authorization code flow with PKCE
Why D: The authorization code flow with PKCE (Proof Key for Code Exchange) is the recommended OAuth 2.0 flow for single-page applications (SPAs) that cannot securely store a client secret. PKCE uses a dynamically generated cryptographic code verifier and challenge, ensuring that even if the authorization code is intercepted, it cannot be exchanged for tokens without the original verifier. This flow is designed for public clients (like SPAs) and avoids the need for custom operational scripts.
Variation 2. A single-page app signs in users with Microsoft Entra ID and calls a protected API. The app cannot safely keep a client secret. Which OAuth flow should be used? The architecture review board prefers a managed Azure-native control.
hard- A.Implicit flow
- B.Client credentials flow
- C.Resource owner password credentials flow
- ✓ D.Authorization code flow with PKCE
Why D: The authorization code flow with PKCE (Proof Key for Code Exchange) is the correct choice because it is designed for public clients (like single-page apps) that cannot securely store a client secret. PKCE uses a dynamically generated cryptographic code verifier and challenge to prevent authorization code interception attacks, making it the recommended OAuth 2.0 flow for SPAs calling protected APIs in Microsoft Entra ID.
Variation 3. A single-page app signs in users with Microsoft Entra ID and calls a protected API. The app cannot safely keep a client secret. Which OAuth flow should be used? The team wants the control to be enforceable during normal operations.
hard- A.Implicit flow
- B.Client credentials flow
- C.Resource owner password credentials flow
- ✓ D.Authorization code flow with PKCE
Why D: The authorization code flow with PKCE (Proof Key for Code Exchange) is the recommended OAuth flow for single-page apps because it prevents the client secret from being exposed by using a dynamically generated code verifier and challenge. This flow ensures that even if the authorization code is intercepted, it cannot be exchanged for tokens without the original code verifier, making it secure for public clients that cannot safely store secrets.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This AZ-204 practice question is part of Courseiva's free Microsoft 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 AZ-204 exam.