Client credentials flow in Rest API - Step-by-Step Execution

Concept Flow - Client credentials flow

Client prepares credentials

↓

Client sends POST request to Auth Server

↓

Auth Server validates credentials

The client sends its credentials to the authorization server, which validates them and returns an access token if valid. The client then uses this token to access protected APIs.

Execution Sample

Rest API

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

grant_type=client_credentials&client_id=abc&client_secret=xyz

Client sends a POST request with client credentials to get an access token.

Execution Table

Step	Action	Request Data	Server Validation	Response
1	Client prepares POST request	grant_type=client_credentials&client_id=abc&client_secret=xyz	N/A	N/A
2	Client sends POST request to Auth Server	Same as above	Server checks client_id and client_secret	N/A
3	Auth Server validates credentials	N/A	Credentials valid?	Yes
4	Auth Server issues access token	N/A	N/A	{"access_token":"token123","token_type":"Bearer","expires_in":3600}
5	Client receives access token	N/A	N/A	Token stored for API calls
6	Client uses token to access API	Authorization: Bearer token123	Server checks token validity	API response data
7	End	N/A	N/A	Process complete

💡 Process ends after client receives token and uses it to access API.

Variable Tracker

Variable	Start	After Step 1	After Step 4	After Step 5	Final
client_id	None	abc	abc	abc	abc
client_secret	None	xyz	xyz	xyz	xyz
access_token	None	None	token123	token123	token123
token_type	None	None	Bearer	Bearer	Bearer

Key Moments - 3 Insights

Why does the client send client_id and client_secret in the request?

The client_id and client_secret prove the client's identity to the authorization server, as shown in execution_table step 2 and 3 where the server validates these credentials.

What happens if the credentials are invalid?

If credentials are invalid, the server returns an error response instead of an access token, as indicated by the 'No' branch in the concept_flow and would appear in execution_table step 3 with a failure response.

Why does the client use the access token after receiving it?

The access token is used to authenticate API requests, allowing the client to access protected resources, as shown in execution_table step 6 where the token is sent in the Authorization header.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of access_token after step 4?

A "token123"

B None

C "abc"

D "Bearer"

💡 Hint

Check the 'Response' column at step 4 where the server issues the access token.

At which step does the server validate the client credentials?

A Step 1

B Step 5

C Step 3

D Step 6

💡 Hint

Look at the 'Server Validation' column in the execution_table.

If the client_secret was wrong, what would change in the execution flow?

A The server would issue a token anyway.

B The server would return an error instead of a token.

C The client would skip sending the POST request.

D The client would receive a token with a different type.

💡 Hint

Refer to the 'No' branch in the concept_flow and the explanation in key_moments about invalid credentials.

Concept Snapshot

Client Credentials Flow:
- Client sends client_id and client_secret to Auth Server.
- Auth Server validates credentials.
- If valid, server returns access token.
- Client uses token to access protected APIs.
- No user interaction involved.
- Used for server-to-server authentication.

Full Transcript

The Client Credentials Flow is a way for a client application to get an access token from an authorization server by sending its own credentials. First, the client prepares a POST request including its client_id and client_secret. Then, it sends this request to the authorization server's token endpoint. The server checks if the credentials are valid. If they are, the server responds with an access token. The client stores this token and uses it in the Authorization header to access protected APIs. If the credentials are invalid, the server returns an error and no token is issued. This flow is used when no user is involved, such as server-to-server communication.

Practice

1. What is the main purpose of the client credentials flow in REST APIs?

easy

A. To allow an application to get an access token by proving its own identity without a user.

B. To authenticate a user with username and password.

C. To refresh an expired access token using a refresh token.

D. To allow users to log in using social media accounts.

Solution

1. Step 1: Understand client credentials flow purpose

   This flow is designed for applications to authenticate themselves, not users.

2. Step 2: Compare options with flow use case

   Only To allow an application to get an access token by proving its own identity without a user. describes the app proving its identity without user involvement.

3. Final Answer:

   To allow an application to get an access token by proving its own identity without a user. -> Option A

4. Quick Check:

   Client credentials flow = app identity only [OK]

Hint: Remember: no user involved, app proves itself [OK]

Common Mistakes:

• Confusing client credentials flow with user login flows
• Thinking refresh tokens are part of this flow
• Assuming social login is related

2. Which HTTP method is typically used to request an access token in the client credentials flow?

easy

A. GET

B. POST

C. PUT

D. DELETE

Solution

1. Step 1: Identify token request method

   Access tokens are requested by sending client ID and secret securely, usually in the request body.

2. Step 2: Match method to secure data sending

   POST method allows sending data in the body securely, unlike GET which sends data in URL.

3. Final Answer:

   POST -> Option B

4. Quick Check:

   Token request uses POST method [OK]

Hint: Token requests send secrets in body, so use POST [OK]

Common Mistakes:

• Using GET which exposes secrets in URL
• Confusing PUT or DELETE with token requests
• Not sending client credentials in request body

3. Given this token request snippet, what is the expected response field containing the access token?

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

grant_type=client_credentials&client_id=abc123&client_secret=secret456

medium

A. "error"

B. "refresh_token"

C. "id_token"

D. "access_token"

Solution

1. Step 1: Understand client credentials response

   The response to this request includes an access token to authorize API calls.

2. Step 2: Identify correct response field

   The field "access_token" holds the token; "refresh_token" and "id_token" are not returned here.

3. Final Answer:

   "access_token" -> Option D

4. Quick Check:

   Access token field = "access_token" [OK]

Hint: Access token always in "access_token" field [OK]

Common Mistakes:

• Expecting a refresh token in client credentials flow
• Confusing id_token with access_token
• Assuming error field means success

4. You wrote this code to request a token but get an error:

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

{"grant_type":"client_credentials","client_id":"abc123","client_secret":"secret456"}

What is the likely cause?

medium

A. Using GET instead of POST method

B. Missing Authorization header with Basic auth

C. Using Content-Type application/json instead of application/x-www-form-urlencoded

D. Incorrect grant_type value

Solution

1. Step 1: Check content type for client credentials flow

   The standard requires sending data as URL-encoded form, not JSON.

2. Step 2: Identify mismatch causing error

   Using application/json causes server to reject request because it expects application/x-www-form-urlencoded.

3. Final Answer:

   Using Content-Type application/json instead of application/x-www-form-urlencoded -> Option C

4. Quick Check:

   Content-Type must be application/x-www-form-urlencoded [OK]

Hint: Use form encoding, not JSON, for client credentials token requests [OK]

Common Mistakes:

• Sending JSON instead of form data
• Omitting required headers
• Using wrong HTTP method

5. You want to securely get an access token for a backend service using client credentials flow. Which of these is the best practice?

hard

A. Send client ID and secret in POST body with Content-Type application/x-www-form-urlencoded over HTTPS

B. Send client ID and secret in HTTP headers without encryption

C. Send client ID and secret in URL query parameters over HTTPS

D. Send client ID and secret in plain text over HTTP

Solution

1. Step 1: Identify secure transmission method

   Client credentials must be sent securely to avoid exposure.

2. Step 2: Choose correct method and protocol

   Sending in POST body with form encoding over HTTPS ensures confidentiality and standard compliance.

3. Final Answer:

   Send client ID and secret in POST body with Content-Type application/x-www-form-urlencoded over HTTPS -> Option A

4. Quick Check:

   Use POST body + HTTPS for secure client credentials [OK]

Hint: Always use POST with HTTPS and form data for client credentials [OK]

Common Mistakes:

• Sending secrets in URL query parameters
• Using HTTP instead of HTTPS
• Sending secrets in headers without encryption