Authorization code flow in Rest API - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of the Authorization Code Flow in OAuth 2.0?

It securely allows a client application to obtain an access token by first getting an authorization code from the authorization server, which is then exchanged for the token.

beginner

List the main steps in the Authorization Code Flow.

1. Client redirects user to authorization server.
2. User logs in and consents.
3. Authorization server sends authorization code to client.
4. Client exchanges code for access token.
5. Client uses access token to access protected resources.

intermediate

Why is the Authorization Code Flow considered more secure than the Implicit Flow?

Because the access token is never exposed to the user agent or browser directly; it is obtained via a secure back-channel between the client and authorization server.

beginner

What role does the 'redirect URI' play in the Authorization Code Flow?

It is the URL where the authorization server sends the authorization code after user consent. It must be pre-registered to prevent attacks.

intermediate

What is the purpose of the 'state' parameter in the Authorization Code Flow?

It helps prevent cross-site request forgery (CSRF) attacks by maintaining state between the request and callback.

In the Authorization Code Flow, what does the client receive first from the authorization server?

A User credentials

B Authorization code

C Refresh token

D Access token

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

Why should the access token not be exposed to the browser in Authorization Code Flow?

A Because tokens are too large

B Because browsers cannot store tokens

C To prevent token theft by malicious scripts

D To speed up the login process

Exposing tokens to the browser risks theft by malicious scripts or attackers.

What is the purpose of exchanging the authorization code for an access token?

A To log out the user

B To get user password

C To refresh the page

D To verify the client identity securely

Exchanging the code for a token ensures the client is authorized securely.

Which parameter helps prevent CSRF attacks in Authorization Code Flow?

A state

B scope

C redirect_uri

D client_id

The 'state' parameter maintains request state and prevents CSRF.

What must the redirect URI be during the Authorization Code Flow?

A Pre-registered with the authorization server

B Randomly generated each time

C The user's homepage

D The client's IP address

The redirect URI must be pre-registered to avoid redirection attacks.

Explain the Authorization Code Flow step-by-step as if teaching a friend.

Think about how the client never sees the user's password and how the token is kept safe.

Describe why the Authorization Code Flow is more secure than other OAuth flows.

Focus on how the flow protects tokens and user data.

Practice

1. What is the main purpose of the authorization code in the Authorization Code Flow?

easy

A. To exchange it for an access token securely

B. To directly access user data

C. To authenticate the user with a password

D. To refresh the access token automatically

Solution

1. Step 1: Understand the role of the authorization code

   The authorization code is a temporary code given after user consent, not the token itself.

2. Step 2: Identify what the app does with the code

   The app sends this code to the authorization server to get an access token securely.

3. Final Answer:

   To exchange it for an access token securely -> Option A

4. Quick Check:

   Authorization code = temporary code for token exchange [OK]

Hint: Authorization code is a temporary code, not a token [OK]

Common Mistakes:

• Thinking the code directly accesses data
• Confusing code with user password
• Assuming code refreshes tokens

2. Which HTTP method is typically used by the app to exchange the authorization code for an access token?

easy

A. DELETE

B. GET

C. PUT

D. POST

Solution

1. Step 1: Recall the token exchange request

   The app sends the authorization code to the token endpoint to get an access token.

2. Step 2: Identify the HTTP method used

   This request uses POST because it sends data securely in the request body.

3. Final Answer:

   POST -> Option D

4. Quick Check:

   Token exchange uses POST method [OK]

Hint: Token exchange sends data securely, so use POST [OK]

Common Mistakes:

• Using GET which exposes data in URL
• Confusing PUT or DELETE with token exchange
• Assuming token exchange is a simple GET request

3. Given this simplified token exchange request in Python:

import requests
response = requests.post('https://auth.example.com/token', data={
    'code': 'abc123',
    'client_id': 'myapp',
    'client_secret': 'secret',
    'redirect_uri': 'https://myapp.com/callback',
    'grant_type': 'authorization_code'
})
print(response.json().get('access_token'))

What will this code print if the exchange is successful?

medium

A. The authorization code 'abc123'

B. The access token string from the server

C. An error message about invalid client

D. None

Solution

1. Step 1: Understand the request purpose

   The code sends a POST request to exchange the authorization code for an access token.

2. Step 2: Analyze the printed output

   If successful, the server returns JSON with an 'access_token' key, which is printed.

3. Final Answer:

   The access token string from the server -> Option B

4. Quick Check:

   response.json()['access_token'] = access token [OK]

Hint: Successful exchange returns access token, not code [OK]

Common Mistakes:

• Printing the code instead of token
• Expecting error message on success
• Not accessing JSON correctly

4. In the Authorization Code Flow, a developer wrote this code snippet to exchange the code:

response = requests.get('https://auth.example.com/token', params={
    'code': 'abc123',
    'client_id': 'myapp',
    'client_secret': 'secret',
    'redirect_uri': 'https://myapp.com/callback',
    'grant_type': 'authorization_code'
})

What is the main issue with this code?

medium

A. Incorrect redirect URI format

B. Missing the authorization code parameter

C. Using GET instead of POST for token exchange

D. Client secret should not be sent

Solution

1. Step 1: Check HTTP method for token exchange

   The token exchange requires a POST request to send sensitive data securely.

2. Step 2: Identify the problem in the code

   The code uses GET with query parameters, which is insecure and not standard for this flow.

3. Final Answer:

   Using GET instead of POST for token exchange -> Option C

4. Quick Check:

   Token exchange must use POST, not GET [OK]

Hint: Token exchange always uses POST, not GET [OK]

Common Mistakes:

• Using GET exposes secrets in URL
• Forgetting to send client secret
• Assuming redirect URI format is wrong

5. A web app uses Authorization Code Flow with PKCE (Proof Key for Code Exchange). Which additional step does PKCE add to improve security?

hard

A. The app sends a code verifier with the token request to prove it initiated the flow

B. The app uses client secret only without authorization code

C. The user enters their password twice during login

D. The app skips the authorization code and uses implicit flow

Solution

1. Step 1: Understand PKCE purpose

   PKCE adds a code verifier and challenge to prevent interception of the authorization code.

2. Step 2: Identify the added step in the flow

   The app sends the code verifier with the token request to prove it started the flow and prevent attacks.

3. Final Answer:

   The app sends a code verifier with the token request to prove it initiated the flow -> Option A

4. Quick Check:

   PKCE adds code verifier step for security [OK]

Hint: PKCE adds code verifier to token request for security [OK]

Common Mistakes:

• Thinking PKCE removes authorization code
• Confusing PKCE with password prompts
• Assuming PKCE uses implicit flow