Why do my browser AJAX calls report missing Access-Control headers while Postman works normally?

Other Questions JavaScript Closures
1

Browser AJAX calls to my Flask-based API report a missing ‘Access-Control-Allow-Origin’ header, yet Postman requests succeed. For instance:

$.ajax({
  url: apiEndpoint,
  method: 'POST',
  credentials: true
})
.done(() => console.log('Request succeeded'))
.fail(() => console.error('Request failed'));
2

In my experience, the issue appears to be with how browsers strictly enforce cross-origin resource sharing. Unlike Postman, browsers perform preflight checks and expect specific headers including Access-Control-Allow-Origin to correctly match the origin of the request. When credentials are involved, browsers also need Access-Control-Allow-Credentials which may not be present in your current configuration. I had a similar problem on a Flask API and resolved it by using middleware to attach the required CORS headers during both preflight and standard requests. Evaluating your server setup and perhaps using flask-cors might help resolve the issue.

3

Based on my experience, this issue can arise when the server does not send back the precise CORS headers required for credentialed requests. In cases where credentials are enabled, the Access-Control-Allow-Origin header must match the request’s origin; using a wildcard will not suffice. I encountered a similar situation using a Flask API and realized that the configuration on the server side was not set to appropriately respond to the browser’s preflight requests. Adjusting the server’s response configuration to include an explicit origin value resolved the problem in my setup.