CORS testing in Postman - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does CORS stand for and why is it important?

CORS stands for Cross-Origin Resource Sharing. It is important because it controls how web pages can request resources from a different domain, protecting users from malicious websites accessing data they shouldn't.

intermediate

What is a 'preflight request' in CORS?

A preflight request is an automatic OPTIONS request sent by the browser before the actual request. It checks if the server allows the real request method and headers from the origin.

beginner

How can you test CORS policies using Postman?

In Postman, you can simulate CORS requests by setting the 'Origin' header manually and sending requests to the server. You check the server's response headers like 'Access-Control-Allow-Origin' to verify CORS behavior.

beginner

What response header indicates that a server allows cross-origin requests?

The 'Access-Control-Allow-Origin' header indicates which origins are allowed to access the resource. If it matches the request origin or is '*', the request is allowed.

intermediate

Why might a CORS request fail even if the server responds?

A CORS request might fail if the server does not include the correct 'Access-Control-Allow-Origin' header or if the preflight OPTIONS request is rejected. Browsers block the response for security.

Which header must a server send to allow cross-origin requests?

A Authorization

B Content-Type

C Access-Control-Allow-Origin

D User-Agent

The 'Access-Control-Allow-Origin' header tells the browser which origins are allowed to access the resource.

What HTTP method is used for a CORS preflight request?

A GET

B POST

C PUT

D OPTIONS

The browser sends an OPTIONS request before the actual request to check if the server allows it.

In Postman, how do you simulate a CORS request?

A By setting the 'Origin' header manually

B By enabling CORS mode in settings

C By using the GET method only

D By disabling SSL verification

Postman does not enforce CORS, so you simulate it by adding the 'Origin' header manually.

What does a wildcard '*' in 'Access-Control-Allow-Origin' mean?

A No origins are allowed

B All origins are allowed

C Only localhost is allowed

D Only secure origins are allowed

A '*' means the server allows requests from any origin.

Why do browsers block CORS requests without proper headers?

A To protect user data from unauthorized access

B To improve page load speed

C To reduce server load

D To enable caching

Browsers block such requests to protect users from malicious websites accessing sensitive data.

Explain how you would test CORS behavior using Postman.

Think about how browsers check CORS and how Postman can mimic that.

Describe what happens during a CORS preflight request and why it is needed.

Consider the safety checks browsers do before sending certain requests.

Practice

1. What is the main purpose of CORS testing in Postman?

easy

A. To verify if a server allows resource sharing from different origins

B. To check if the server is running the latest software version

C. To test the speed of the API response

D. To validate the syntax of the API request

Solution

1. Step 1: Understand CORS concept

   CORS (Cross-Origin Resource Sharing) controls if a server permits requests from different websites (origins).

2. Step 2: Relate to Postman testing

   In Postman, testing CORS means checking if the server allows resource sharing from other origins.

3. Final Answer:

   To verify if a server allows resource sharing from different origins -> Option A

4. Quick Check:

   CORS testing = verify cross-origin sharing [OK]

Hint: CORS is about sharing resources across websites [OK]

Common Mistakes:

• Confusing CORS with API speed testing
• Thinking CORS checks server software version
• Assuming CORS validates request syntax

2. Which header should you set in Postman to simulate a cross-origin request for CORS testing?

easy

A. Accept-Encoding

B. Authorization

C. Content-Type

D. Origin

Solution

1. Step 1: Identify header for cross-origin requests

   The Origin header tells the server which website is making the request.

2. Step 2: Use in Postman

   Setting the Origin header in Postman simulates a request from that origin for CORS testing.

3. Final Answer:

   Origin -> Option D

4. Quick Check:

   Set Origin header to test CORS [OK]

Hint: Set 'Origin' header to mimic browser cross-site requests [OK]

Common Mistakes:

• Using Authorization header instead of Origin
• Confusing Content-Type with Origin
• Not setting any header for CORS simulation

3. You send a request in Postman with header Origin: https://example.com. The server responds with header Access-Control-Allow-Origin: https://example.com. What does this response indicate?

medium

A. The server allows requests only from https://example.com

B. The server has a syntax error in headers

C. The server allows requests from any origin

D. The server denies requests from https://example.com

Solution

1. Step 1: Understand Access-Control-Allow-Origin header

   This header specifies which origin the server allows for cross-origin requests.

2. Step 2: Match response header with request origin

   The server's response matches the request origin exactly, meaning it allows requests only from that origin.

3. Final Answer:

   The server allows requests only from https://example.com -> Option A

4. Quick Check:

   Matching Access-Control-Allow-Origin means allowed origin [OK]

Hint: Matching Access-Control-Allow-Origin means allowed origin [OK]

Common Mistakes:

• Thinking it denies the origin when it matches
• Assuming wildcard '*' when specific origin is set
• Confusing syntax error with valid header

4. You test CORS in Postman by setting Origin: https://myapp.com, but the server response lacks Access-Control-Allow-Origin header. What is the likely issue?

medium

A. The request method is invalid

B. The server does not support CORS or is misconfigured

C. Postman does not send the Origin header correctly

D. The server is down

Solution

1. Step 1: Check server response headers for CORS

   If Access-Control-Allow-Origin is missing, the server likely does not allow cross-origin requests.

2. Step 2: Rule out Postman or request issues

   Postman sends headers correctly; missing header usually means server misconfiguration or no CORS support.

3. Final Answer:

   The server does not support CORS or is misconfigured -> Option B

4. Quick Check:

   Missing Access-Control-Allow-Origin means no CORS support [OK]

Hint: Missing Access-Control-Allow-Origin means server blocks CORS [OK]

Common Mistakes:

• Blaming Postman for missing headers
• Assuming request method causes missing header
• Thinking server is down without checking status

5. You want to test if a server allows CORS requests from multiple origins using Postman. Which approach is best to verify this?

hard

A. Send requests with random headers and check response time

B. Send one request without Origin header and check server status

C. Send multiple requests with different Origin headers and check Access-Control-Allow-Origin in each response

D. Send requests only from localhost and assume all origins are allowed

Solution

1. Step 1: Understand testing multiple origins

   To test multiple origins, you must simulate requests from each origin by setting the Origin header accordingly.

2. Step 2: Verify server response for each origin

   Check if Access-Control-Allow-Origin matches the requested origin or allows all origins (wildcard).

3. Final Answer:

   Send multiple requests with different Origin headers and check Access-Control-Allow-Origin in each response -> Option C

4. Quick Check:

   Test each origin with Origin header and check response [OK]

Hint: Test each origin separately by changing Origin header [OK]

Common Mistakes:

• Testing without Origin header
• Assuming localhost tests all origins
• Checking unrelated headers or response time