OAuth 2.0 overview in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary purpose of OAuth 2.0?

OAuth 2.0 is widely used in web applications. What is its main goal?

A To provide a database for storing user credentials securely

B To encrypt all data sent between client and server

C To allow users to share their private resources stored on one site with another site without sharing credentials

D To replace passwords with biometric authentication

💡 Hint

Think about how apps let you log in using accounts from other services without giving your password.

OAuth 2.0 allows users to grant limited access to their resources on one site to another site without sharing their passwords. It acts as a secure authorization framework.

❓ Predict Output

intermediate

What is the output of this OAuth 2.0 token request response?

Given this JSON response from an OAuth 2.0 token endpoint, what is the value of the expires_in field?

Rest API

{
  "access_token": "abc123xyz",
  "token_type": "Bearer",
  "expires_in": 3600,
  "refresh_token": "def456uvw"
}

A 3600

B "Bearer"

C "abc123xyz"

D "def456uvw"

💡 Hint

Look for the field that tells how long the access token is valid in seconds.

The expires_in field indicates the lifetime of the access token in seconds. Here, it is 3600 seconds (1 hour).

🔧 Debug

advanced

Why does this OAuth 2.0 authorization code flow fail?

Consider this simplified OAuth 2.0 authorization code flow snippet. Why does it fail to obtain an access token?

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

client_id=abc&redirect_uri=https://app.example.com/callback&code=xyz123

A The <code>code</code> parameter is invalid because it is not URL encoded

B The <code>redirect_uri</code> should not be included in the token request

C Incorrect HTTP method; should be GET instead of POST

D Missing <code>client_secret</code> parameter in the token request

💡 Hint

Think about what the authorization server needs to verify the client identity during token exchange.

In the authorization code flow, the client must send its client_secret along with the client_id to authenticate itself when exchanging the code for an access token. Omitting it causes failure.

📝 Syntax

advanced

Which option correctly represents a valid OAuth 2.0 Bearer token HTTP header?

Choose the correct syntax for sending an OAuth 2.0 Bearer token in an HTTP request header.

A Authorization: Bearer abc123xyz

B Auth: Bearer abc123xyz

C Authorization: Token abc123xyz

D Bearer: Authorization abc123xyz

💡 Hint

The standard header name and scheme must be used exactly.

The correct header is Authorization with the scheme Bearer followed by the token. Other forms are invalid or non-standard.

🚀 Application

expert

How many scopes are granted in this OAuth 2.0 access token response?

Given this OAuth 2.0 token response, how many scopes does the access token have?

{
  "access_token": "token123",
  "token_type": "Bearer",
  "expires_in": 1800,
  "scope": "read write delete"
}

A 1

B 3

C 0

D 4

💡 Hint

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

The scope field lists permissions separated by spaces. Here, there are three: read, write, and delete.

Practice

1. What is the main purpose of OAuth 2.0 in REST APIs?

easy

A. To replace usernames with email addresses

B. To encrypt all data sent between client and server

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

D. To speed up API response times

Solution

1. Step 1: Understand OAuth 2.0's role

   OAuth 2.0 is designed to let apps access user data safely without needing the user's password.

2. Step 2: Compare options to OAuth 2.0 purpose

   Only To allow apps to access user data securely without sharing passwords correctly describes this purpose. Options A, B, and D describe unrelated functions.

3. Final Answer:

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

4. Quick Check:

   OAuth 2.0 = Secure data access without password sharing [OK]

Hint: OAuth 2.0 = safe access without password sharing [OK]

Common Mistakes:

• Confusing OAuth with encryption protocols
• Thinking OAuth replaces usernames
• Assuming OAuth speeds up APIs

2. Which of the following is the correct OAuth 2.0 flow step to get an access token?

easy

A. Client sends password directly to resource server

B. Client sends authorization code to the authorization server

C. Resource server sends access token to client without request

D. Client sends refresh token to user

Solution

1. Step 1: Identify OAuth 2.0 token exchange step

   The client sends the authorization code to the authorization server to exchange it for an access token.

2. Step 2: Eliminate incorrect options

   Client sends password directly to resource server is wrong because passwords are not sent directly. Resource server sends access token to client without request is wrong because tokens are sent after request. Client sends refresh token to user is wrong because refresh tokens are sent to the authorization server, not the user.

3. Final Answer:

   Client sends authorization code to the authorization server -> Option B

4. Quick Check:

   Authorization code sent to server = Step to get access token [OK]

Hint: Authorization code sent to server to get token [OK]

Common Mistakes:

• Sending password instead of authorization code
• Expecting tokens without request
• Confusing refresh token recipient

3. Given this OAuth 2.0 flow snippet:

1. Client requests authorization code
2. User grants permission
3. Client receives authorization code
4. Client sends authorization code to token endpoint
5. Token endpoint returns access token

What is the output after step 5?

medium

A. Client has an access token to access protected resources

B. Client has the user's password

C. Client can directly access user data without token

D. Client must request authorization code again

Solution

1. Step 1: Follow OAuth 2.0 flow steps

   After step 5, the client receives an access token from the token endpoint.

2. Step 2: Understand access token purpose

   The access token lets the client access protected user data securely without needing the password.

3. Final Answer:

   Client has an access token to access protected resources -> Option A

4. Quick Check:

   Access token received = Access to resources [OK]

Hint: Access token means access granted to resources [OK]

Common Mistakes:

• Thinking client gets user password
• Assuming token is not needed for access
• Believing authorization code must be requested again

4. Identify the error in this OAuth 2.0 flow:

Client sends access token directly to user
User sends authorization code to resource server

medium

A. Access token should be sent to resource server, not user

B. Authorization code should be sent to client, not user

C. Client should never send tokens at all

D. User should send access token to authorization server

Solution

1. Step 1: Analyze token flow roles

   Access tokens are meant for the resource server to verify access, not for the user.

2. Step 2: Check authorization code flow

   The authorization code is sent from user to client, not to the resource server.

3. Final Answer:

   Access token should be sent to resource server, not user -> Option A

4. Quick Check:

   Access token destination = Resource server [OK]

Hint: Access token goes to resource server, not user [OK]

Common Mistakes:

• Sending access token to user instead of server
• Confusing authorization code recipient
• Thinking client never sends tokens

5. You want to build an app that accesses user data from a REST API using OAuth 2.0. Which combination correctly describes the roles and tokens involved?

hard

A. Client app sends refresh token to user to renew access token

B. User sends access token to client app, which then sends password to resource server

C. Resource server issues authorization code directly to client app without user consent

D. Client app uses authorization code to get access token from authorization server, then uses access token to access resource server

Solution

1. Step 1: Understand OAuth 2.0 roles

   The client app requests an authorization code from the authorization server after user consent.

2. Step 2: Token exchange and usage

   The client exchanges the authorization code for an access token, then uses it to access the resource server.

3. Final Answer:

   Client app uses authorization code to get access token from authorization server, then uses access token to access resource server -> Option D

4. Quick Check:

   Authorization code -> access token -> resource access [OK]

Hint: Authorization code to token, then token to resource [OK]

Common Mistakes:

• Thinking user sends tokens to client
• Assuming resource server issues codes without user
• Confusing refresh token flow