Client credentials flow in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this OAuth 2.0 client credentials flow request?

Given the following HTTP POST request to obtain an access token using client credentials flow, what is the expected JSON response body?

POST /oauth2/token HTTP/1.1
Host: auth.example.com
Content-Type: application/x-www-form-urlencoded

grant_type=client_credentials&client_id=abc123&client_secret=secretXYZ&scope=read

A {"error":"invalid_grant","error_description":"The provided authorization grant is invalid."}

B {"error":"invalid_client","error_description":"Client authentication failed."}

C {"access_token":"abc123secretXYZ","token_type":"Basic","expires_in":3600}

D {"access_token":"eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...","token_type":"Bearer","expires_in":3600,"scope":"read"}

💡 Hint

In client credentials flow, a successful response returns an access token with token type Bearer.

The client credentials flow returns a JSON with an access token, token type Bearer, expiration time, and the requested scope. Errors like invalid_client or invalid_grant indicate failure.

🧠 Conceptual

intermediate

Which statement best describes the client credentials flow?

Choose the correct description of the OAuth 2.0 client credentials flow.

A It is a flow where the client exchanges an authorization code for an access token.

B It is used by applications to obtain an access token by authenticating themselves without user involvement.

C It requires the user to provide a username and password directly to the client application.

D It allows a user to grant a third-party app access to their resources by logging in interactively.

💡 Hint

Think about whether a user is involved in the client credentials flow.

The client credentials flow is designed for server-to-server communication where the client app authenticates itself directly without user interaction.

🔧 Debug

advanced

Why does this client credentials request fail with invalid_client error?

Examine the following HTTP request and identify why the server responds with an invalid_client error.

POST /oauth2/token HTTP/1.1
Host: auth.example.com
Content-Type: application/x-www-form-urlencoded

client_id=abc123&client_secret=secretXYZ&grant_type=client_credentials

A The client_id and client_secret are missing from the request body.

B The Content-Type header is incorrect; it should be application/json.

C The client credentials must be sent in the Authorization header, not in the body.

D The grant_type parameter is missing or misspelled.

💡 Hint

Check how client authentication is typically done in OAuth 2.0 client credentials flow.

OAuth 2.0 recommends sending client credentials in the Authorization header using Basic authentication. Sending them in the body may cause invalid_client errors depending on server configuration.

📝 Syntax

advanced

Which HTTP request correctly implements client credentials flow?

Choose the correctly formed HTTP POST request to obtain an access token using client credentials flow.

A

POST /oauth2/token HTTP/1.1
Host: auth.example.com
Content-Type: application/x-www-form-urlencoded
Authorization: Basic YWJjMTIzOnNlY3JldFlZWg==

grant_type=client_credentials

B

POST /oauth2/token HTTP/1.1
Host: auth.example.com
Content-Type: application/json

{"client_id":"abc123","client_secret":"secretXYZ","grant_type":"client_credentials"}

C

POST /oauth2/token HTTP/1.1
Host: auth.example.com
Content-Type: application/x-www-form-urlencoded

client_id=abc123&client_secret=secretXYZ&grant_type=client_credentials

D

GET /oauth2/token?client_id=abc123&client_secret=secretXYZ&grant_type=client_credentials HTTP/1.1
Host: auth.example.com

💡 Hint

Remember the HTTP method and header requirements for client credentials flow.

The client credentials flow requires a POST request with Content-Type application/x-www-form-urlencoded and client credentials in the Authorization header using Basic authentication.

🚀 Application

expert

How many scopes are granted in this client credentials token response?

Given this JSON response from a client credentials token request, how many scopes does the access token have?

{
  "access_token": "eyJz93a...k4laUWw",
  "token_type": "Bearer",
  "expires_in": 3600,
  "scope": "read write delete"
}

A 3

B 2

C 1

D 4

💡 Hint

Count the number of space-separated scopes in the scope string.

The scope string contains three scopes separated by spaces: read, write, and delete.