OAuth 2.0 flow in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of OAuth 2.0?

OAuth 2.0 is a protocol that allows a user to grant limited access to their resources on one site to another site, without sharing their credentials.

beginner

Name the four main roles in the OAuth 2.0 flow.

The four main roles are: Resource Owner (user), Client (app requesting access), Authorization Server (issues tokens), and Resource Server (hosts protected resources).

beginner

What is an access token in OAuth 2.0?

An access token is a credential issued by the authorization server that the client uses to access protected resources on behalf of the user.

intermediate

Explain the Authorization Code Grant flow in OAuth 2.0.

The client redirects the user to the authorization server to get an authorization code. Then the client exchanges this code for an access token to access resources securely.

intermediate

Why is the Authorization Code Grant flow considered more secure than the Implicit Grant?

Because the access token is never exposed to the user agent or browser directly; it is exchanged securely on the backend, reducing risk of token leakage.

Which OAuth 2.0 role is responsible for issuing access tokens?

A Resource Server

B Client

C Resource Owner

D Authorization Server

The Authorization Server issues access tokens after authenticating the user and client.

In OAuth 2.0, what does the client receive after the user authorizes access in the Authorization Code Grant?

A Access token directly

B Authorization code

C Refresh token only

D User credentials

The client first receives an authorization code, which it exchanges for an access token.

Which OAuth 2.0 flow is best suited for single-page applications without a backend?

A Implicit Grant

B Client Credentials Grant

C Authorization Code Grant

D Resource Owner Password Credentials Grant

Implicit Grant is designed for apps that cannot securely store client secrets, like single-page apps.

What is the main security risk if the access token is exposed in the browser?

A Client secret exposure

B User password theft

C Token replay or misuse

D Authorization code leakage

If an access token is exposed, attackers can reuse it to access protected resources.

Which OAuth 2.0 role hosts the protected resources the client wants to access?

A Resource Server

B Resource Owner

C Client

D Authorization Server

The Resource Server holds the protected data and validates access tokens.

Describe the step-by-step flow of the OAuth 2.0 Authorization Code Grant.

Think about how the client gets permission and then accesses resources.

Explain why OAuth 2.0 improves security compared to sharing user credentials directly.

Focus on how OAuth limits what the client can do and protects user passwords.

Practice

1. What is the main purpose of the OAuth 2.0 Authorization Code flow?

easy

A. To replace user authentication with biometric data

B. To store user passwords safely on the server

C. To allow apps to access user data securely without sharing passwords

D. To encrypt all user data before sending it

Solution

1. Step 1: Understand OAuth 2.0 Authorization Code flow

   This flow lets apps get permission to access user data without needing the user's password directly.

2. Step 2: Identify the main goal

   The goal is to keep user credentials safe while allowing apps to act on behalf of the user.

3. Final Answer:

   To allow apps to access user data securely without sharing passwords -> Option C

4. Quick Check:

   OAuth 2.0 Authorization Code flow = Secure app access without passwords [OK]

Hint: Remember: OAuth avoids sharing passwords directly [OK]

Common Mistakes:

• Confusing OAuth with password storage
• Thinking OAuth encrypts all user data
• Assuming OAuth replaces authentication methods

2. Which of the following is the correct sequence in the OAuth 2.0 Authorization Code flow?

easy

A. User logs in -> App receives access token -> App requests authorization code

B. App sends password -> User grants access -> App receives token

C. App directly receives access token -> User logs in -> Access granted

D. User logs in -> App gets authorization code -> App exchanges code for access token

Solution

1. Step 1: Recall the OAuth 2.0 Authorization Code flow steps

   User logs in and authorizes the app, which then receives an authorization code.

2. Step 2: Understand token exchange

   The app exchanges the authorization code for an access token to access user data.

3. Final Answer:

   User logs in -> App gets authorization code -> App exchanges code for access token -> Option D

4. Quick Check:

   Authorization Code flow sequence = Login, code, token [OK]

Hint: Authorization code comes before access token [OK]

Common Mistakes:

• Thinking app gets token before authorization code
• Confusing password sending with OAuth flow
• Mixing order of login and token receipt

3. Given this OAuth 2.0 flow snippet, what is the expected output after exchanging the authorization code?

POST /token
Headers: Content-Type: application/x-www-form-urlencoded
Body: grant_type=authorization_code&code=abc123&redirect_uri=https://app.com/callback&client_id=client123&client_secret=secretXYZ

Response:
{
  "access_token": "token456",
  "token_type": "Bearer",
  "expires_in": 3600
}

medium

A. An access token 'token456' valid for 3600 seconds

B. An error because client_secret is missing

C. A refresh token instead of access token

D. No token, only a confirmation message

Solution

1. Step 1: Analyze the token request

   The app sends a POST request with grant_type authorization_code and includes the code and client credentials.

2. Step 2: Interpret the response

   The response contains an access_token 'token456', token_type 'Bearer', and expires_in 3600 seconds, meaning the token is valid for one hour.

3. Final Answer:

   An access token 'token456' valid for 3600 seconds -> Option A

4. Quick Check:

   Token response includes access_token and expiry [OK]

Hint: Token response includes access_token and expiry time [OK]

Common Mistakes:

• Expecting refresh token instead of access token
• Missing client_secret causes error
• Confusing confirmation message with token response

4. A developer implements OAuth 2.0 Authorization Code flow but the app never receives an access token after exchanging the code. What is the most likely cause?

medium

A. The app forgot to include the client_id in the authorization request

B. The redirect URI used in the token request does not match the one registered

C. The user did not enter their password correctly

D. The access token expired before the request

Solution

1. Step 1: Identify common OAuth token exchange errors

   One frequent error is a mismatch in redirect URI between authorization and token requests.

2. Step 2: Understand impact of redirect URI mismatch

   If the redirect URI differs, the authorization server rejects the token request, so no access token is returned.

3. Final Answer:

   The redirect URI used in the token request does not match the one registered -> Option B

4. Quick Check:

   Redirect URI mismatch blocks token issuance [OK]

Hint: Check redirect URI matches exactly in all requests [OK]

Common Mistakes:

• Blaming user password errors for token issues
• Ignoring client_id in authorization request
• Assuming token expires before it is issued

5. You are designing a system that uses OAuth 2.0 Authorization Code flow for a mobile app and a web app. Which approach best secures the access tokens and refresh tokens?

hard

A. Use Authorization Code flow with PKCE for the mobile app and standard Authorization Code flow for the web app

B. Use Implicit flow for both apps to simplify token handling

C. Store access tokens in local storage for both apps to allow easy access

D. Send access tokens directly in URL query parameters for both apps

Solution

1. Step 1: Understand security needs for mobile and web apps

   Mobile apps cannot keep client secrets safe, so PKCE (Proof Key for Code Exchange) enhances security for Authorization Code flow.

2. Step 2: Choose best practices for token storage and flow

   Web apps can use standard Authorization Code flow with client secret. Implicit flow is deprecated due to security risks. Storing tokens in local storage or URLs exposes them to attacks.

3. Final Answer:

   Use Authorization Code flow with PKCE for the mobile app and standard Authorization Code flow for the web app -> Option A

4. Quick Check:

   PKCE secures mobile OAuth; standard flow suits web apps [OK]

Hint: Use PKCE for mobile apps, standard flow for web apps [OK]

Common Mistakes:

• Using Implicit flow despite security risks
• Storing tokens insecurely in local storage
• Passing tokens in URL parameters