Why CORS Errors Occur in JavaScript but Not Postman

The error occurs because browsers enforce CORS (Cross-Origin Resource Sharing) restrictions as part of the same-origin policy to prevent malicious cross-site requests, while Postman bypasses these browser security mechanisms entirely. CORS is implemented client-side by browsers, not server-side, meaning your Flask API might be working perfectly fine - it’s the browser that’s blocking the response when it doesn’t contain the proper Access-Control-Allow-Origin headers.

Contents

• Understanding Browser Security: The Same-Origin Policy
• How CORS Actually Works in Browsers
• Why Postman and Other Tools Don’t Have CORS Issues
• Technical Flow: Browser vs. Postman Request Processing
• The Role of Preflight Requests in CORS
• Browser-Specific CORS Implementation Details

Understanding Browser Security: The Same-Origin Policy

The same-origin policy is a fundamental security mechanism built into all modern browsers that prevents scripts on one page from accessing data on another page with a different origin. An origin is defined by the combination of protocol (HTTP/HTTPS), hostname, and port number. For example, https://example.com:8080 has a different origin than https://example.com:3000 due to the different ports, and both are different from http://example.com due to the different protocol.

According to Wikipedia, “This standard extends HTTP with a new Origin request header and a new Access-Control-Allow-Origin response header. It allows servers to use a header to explicitly list origins that may request a file or to use a wildcard and allow a file to be requested by any site.”

This security measure was developed to prevent malicious websites from making requests to user’s other accounts (like banking or email) without their knowledge. When your JavaScript code tries to fetch resources from a different origin, it becomes a cross-origin HTTP request, which browsers block by default unless the server explicitly permits it through CORS headers.

How CORS Actually Works in Browsers

CORS is not a server-side technology but rather a browser-enforced security protocol. When you make a cross-origin request from JavaScript, here’s what happens:

1. Your browser sends the HTTP request to the server
2. The server processes the request and sends back a response
3. Before delivering the response to your JavaScript code, the browser checks the response headers
4. If the response doesn’t include the required Access-Control-Allow-Origin header (or doesn’t include your origin specifically), the browser blocks access to the response
5. Your JavaScript code never receives the response - instead, you get a CORS error

The key insight here is that the server response is actually received successfully - it’s the browser that’s preventing your JavaScript from accessing it. As Sentry explains, “CORS does not protect a resource, such as an API endpoint, against unwanted access. CORS is implemented by browsers on the client side.”

This is why you can see the same response working perfectly in Postman but failing in your browser - the server is sending identical responses in both cases, but browsers have additional security layers that other tools don’t implement.

Why Postman and Other Tools Don’t Have CORS Issues

Postman does not enforce CORS restrictions because it’s not a browser - it’s a standalone application that doesn’t implement the same-origin policy. Several key differences explain why Postman works when browsers fail:

Postman’s Security Model

Postman operates as a desktop application that runs outside the browser’s security sandbox. When you make a request in Postman, it’s essentially making a server-to-server request, which is not subject to the same-origin policy restrictions that browsers enforce.

Browser vs. Application Security Context

• Browsers: Designed to run untrusted code from multiple origins in a shared environment, requiring strict security boundaries
• Postman: Runs as a trusted application with direct network access, bypassing browser security mechanisms

Request Processing Differences

As noted in the Reddit discussion, “Server <-> Server requests (or Postman in your case) is not affected.” Postman doesn’t need to worry about cross-origin scripting attacks because it’s not executing arbitrary web code from different origins.

Technical Flow: Browser vs. Postman Request Processing

Let’s examine the technical differences in how browsers and Postman process the exact same request:

Browser Request Processing Flow

1. Origin Check: Browser detects the request is cross-origin
2. Preflight Request: For certain request types (like POST with credentials), browser sends an OPTIONS request first
3. CORS Validation: Browser checks server’s CORS headers in the response
4. Main Request: Only if preflight succeeds, browser sends the actual request
5. Response Blocking: Browser checks final response for proper CORS headers before allowing JavaScript access

Postman Request Processing Flow

1. Direct Request: Postman sends the HTTP request directly
2. Response Processing: Postman receives and displays the response
3. No CORS Checks: No validation of CORS headers occurs

Critical Technical Difference

The crucial point is that browsers implement CORS as a security layer on top of HTTP, while Postman implements a pure HTTP client without browser security considerations. As the Stack Overflow answer explains, “Browser security stops the cross origin requests. If you disable the chrome security it will do any CORS request just fine.”

The Role of Preflight Requests in CORS

For certain types of cross-origin requests, browsers automatically send a preflight OPTIONS request before making the actual request. This is particularly relevant to your authentication example.

When Preflight Requests Occur

Preflight requests are triggered when:

• Using methods other than GET, HEAD, or POST
• Using POST with Content-Type other than application/x-www-form-urlencoded, multipart/form-data, or text/plain
• Including custom headers
• Using credentials

Your Authentication Request Analysis

Your jQuery ajax setup:

$.ajax({
  type: 'POST',
  dataType: 'text',
  url: api,
  username: 'user',
  password: 'pass',
  crossDomain: true,
  xhrFields: {
    withCredentials: true,
  }
})

This request triggers a preflight because:

1. It’s a POST request with credentials (withCredentials: true)
2. It likely includes custom headers (authentication headers)

The browser first sends an OPTIONS request asking if the server accepts cross-origin requests with these specific parameters. If your Flask server doesn’t handle OPTIONS requests or doesn’t return the correct CORS headers, the browser blocks the actual POST request before it’s even sent.

Browser-Specific CORS Implementation Details

Different browsers implement CORS slightly differently, but the core security principle remains the same:

Origin Validation

Browsers strictly validate that the Access-Control-Allow-Origin header either:

• Lists the exact origin making the request, or
• Uses the wildcard * (though this doesn’t work with credentials)

Error Visibility

Browsers intentionally make CORS errors visible to developers through the console and prevent access to the response data, even though the network request succeeded.

Security Context

Browsers operate in a security sandbox where they must protect users from malicious cross-site scripting attacks. This is why they enforce CORS so strictly - it’s not about the server’s security, but about protecting end-users.

As the tutorialspoint article explains, “Web pages can use the XMLHttpRequest object to send and receive data from remote servers, but they’re limited by the same origin policy but the extensions like Postman are not so limited.”

Sources

1. Sentry - Why does my JavaScript code receive a “No Access-Control-Allow-Origin header is present on the requested resource” error, while Postman does not?
2. Stack Overflow - POSTing to external API throws CORS but it works from Postman
3. Wikipedia - Same-origin policy
4. Reddit - Blocked by CORS on the browser but endpoint working correctly in Postman
5. Tutorialspoint - Why doesn’t Postman get a “No ‘Access-Control-Allow-Origin’ header is present on the requested resource” error in JavaScript
6. CORS Errors FAQ - The browser may well have received exactly the same response that you see with Postman or cURL
7. Stack Overflow - Why does my JavaScript code receive a “No ‘Access-Control-Allow-Origin’ header is present on the requested resource” error, while Postman does not?
8. Refinitiv Developer Community - No ‘Access-Control-Allow-Origin’ header is present on the requested resource (Working in Postman but not in JavaScript)

Conclusion

The fundamental difference between browser JavaScript requests and Postman comes down to security context: browsers enforce CORS as part of their same-origin policy to protect users from malicious cross-site attacks, while Postman operates outside these browser security constraints.

Key takeaways:

• CORS is browser-enforced, not server-enforced - your Flask API is likely working perfectly
• Browsers block responses that don’t include proper CORS headers, even though the network request succeeded
• Postman bypasses browser security because it’s a desktop application, not a browser
• Preflight requests are often the hidden culprit in authentication scenarios
• Server-to-server requests (like Postman makes) are not subject to CORS restrictions

Understanding this distinction is crucial because it tells you where to focus your efforts - the solution isn’t about changing how you make requests from JavaScript, but about configuring your Flask server to include the proper CORS headers that browsers require.