OAuth 2.0 flow in Postman - Build an Automation Script

Test OAuth 2.0 Authorization Code Flow in Postman

Preconditions (3)

Step 1: Open Postman and create a new request

Step 2: Go to the Authorization tab

Step 3: Select OAuth 2.0 as the type

Step 4: Click on Get New Access Token

Step 5: Fill in the fields: Token Name, Auth URL, Access Token URL, Client ID, Client Secret, Scope, Redirect URL

Step 6: Click Request Token

Step 7: Complete the authorization in the browser popup

Step 8: After receiving the token, click Use Token

Step 9: Send the request to the protected resource endpoint

✅ Expected Result: The request is sent with a valid OAuth 2.0 access token in the Authorization header, and the server responds with a successful status code (e.g., 200 OK) and expected data.

Automation Requirements - Postman Test Scripts

Assertions Needed:

Verify response status code is 200

Verify response body contains expected data

Verify Authorization header contains Bearer token

Best Practices:

Use environment variables for client ID, client secret, and URLs

Use Postman pre-request scripts to handle token refresh if needed

Use test scripts to assert response correctness

Avoid hardcoding sensitive data in requests

Automated Solution

Postman

pm.test('Status code is 200', function () {
    pm.response.to.have.status(200);
});

pm.test('Response has expected data', function () {
    const jsonData = pm.response.json();
    pm.expect(jsonData).to.have.property('data');
});

pm.test('Authorization header contains Bearer token', function () {
    const authHeader = pm.request.headers.get('Authorization');
    pm.expect(authHeader).to.match(/^Bearer\s.+$/);
});

The first test checks that the server responded with a 200 OK status, confirming the request succeeded.

The second test parses the JSON response and verifies it contains a property named 'data', which represents expected content from the protected resource.

The third test ensures the Authorization header in the request includes a Bearer token, confirming the OAuth 2.0 token was sent correctly.

Using environment variables and pre-request scripts in Postman helps keep sensitive data secure and automates token management.

Common Mistakes - 3 Pitfalls

Hardcoding client secret and tokens directly in the request

Not verifying the response status code before checking response body

Ignoring token expiration and not refreshing tokens

Bonus Challenge

Now add data-driven testing by running the OAuth 2.0 flow with three different sets of client credentials and scopes

Show Hint

Practice

1. What is the primary purpose of the OAuth 2.0 flow in Postman?

easy

A. To create user accounts automatically

B. To encrypt API requests for better security

C. To generate random API keys for testing

D. To securely authorize access to APIs without sharing user credentials

Solution

1. Step 1: Understand OAuth 2.0 role

   OAuth 2.0 is designed to allow applications to access resources on behalf of a user without exposing their password.

2. Step 2: Identify Postman's use of OAuth 2.0

   Postman uses OAuth 2.0 flow to get access tokens that authorize API calls securely.

3. Final Answer:

   To securely authorize access to APIs without sharing user credentials -> Option D

4. Quick Check:

   OAuth 2.0 = Secure API authorization [OK]

Hint: OAuth 2.0 is about authorization, not encryption or keys [OK]

Common Mistakes:

• Confusing OAuth with encryption
• Thinking OAuth generates API keys
• Assuming OAuth creates user accounts

2. Which of the following is the correct way to set the OAuth 2.0 token URL in Postman?

easy

A. http//api.example.com/oauth/token

B. api.example.com/oauth/token

C. https://api.example.com/oauth/token

D. https://api.example.com/oauth/token/

Solution

1. Step 1: Check URL format

   The token URL must be a full valid URL starting with https:// for security.

2. Step 2: Validate options

   https://api.example.com/oauth/token is a full valid URL with https and no trailing slash, which is standard.

3. Final Answer:

   https://api.example.com/oauth/token -> Option C

4. Quick Check:

   Full HTTPS URL = Correct token URL [OK]

Hint: Always use full HTTPS URL for token endpoint [OK]

Common Mistakes:

• Omitting https:// prefix
• Using incorrect URL syntax
• Adding unnecessary trailing slash

3. In Postman, after configuring OAuth 2.0 with client ID, client secret, and token URL, what will happen when you click Get New Access Token?

medium

A. Postman sends a request to the token URL and retrieves an access token if credentials are valid

B. Postman creates a new user account automatically

C. Postman encrypts the client secret and saves it locally without sending a request

D. Postman resets all environment variables

Solution

1. Step 1: Understand the Get New Access Token button

   This button triggers Postman to request an access token from the OAuth server using provided credentials.

2. Step 2: Identify expected behavior

   If credentials are valid, the server returns an access token which Postman stores for API calls.

3. Final Answer:

   Postman sends a request to the token URL and retrieves an access token if credentials are valid -> Option A

4. Quick Check:

   Get New Access Token = Request token from server [OK]

Hint: Get New Access Token requests token from server [OK]

Common Mistakes:

• Thinking it creates user accounts
• Assuming it only encrypts data locally
• Confusing it with environment reset

4. You configured OAuth 2.0 in Postman but get an error: invalid_client. What is the most likely cause?

medium

A. Token URL is missing https:// prefix

B. Incorrect client ID or client secret provided

C. Access token expired

D. Postman environment variables are empty

Solution

1. Step 1: Analyze the error message

   The error invalid_client means the OAuth server rejected the client credentials.

2. Step 2: Identify common causes

   Most often this happens when client ID or secret is wrong or mistyped.

3. Final Answer:

   Incorrect client ID or client secret provided -> Option B

4. Quick Check:

   invalid_client = Wrong client credentials [OK]

Hint: Check client ID and secret first on invalid_client error [OK]

Common Mistakes:

• Assuming token expiration causes invalid_client
• Ignoring https:// in token URL
• Blaming environment variables without checking credentials

5. You want to automate API testing in Postman using OAuth 2.0. Which approach correctly handles token expiration during tests?

hard

A. Use a pre-request script to check token expiry and request a new token automatically

B. Manually get a new token before each test run

C. Hardcode the access token in headers and never refresh it

D. Disable OAuth and use basic authentication instead

Solution

1. Step 1: Understand token expiration problem

   Access tokens expire, so tests must handle refreshing tokens automatically to avoid failures.

2. Step 2: Identify automation solution in Postman

   Using a pre-request script to check token expiry and request a new token ensures tests always have valid tokens.

3. Final Answer:

   Use a pre-request script to check token expiry and request a new token automatically -> Option A

4. Quick Check:

   Automate token refresh with pre-request script [OK]

Hint: Automate token refresh with pre-request scripts [OK]

Common Mistakes:

• Manually refreshing tokens slows automation
• Hardcoding tokens causes failures on expiry
• Switching auth methods ignores OAuth benefits