How to Fix the \'Request Header Field Access-Control-Allow-Headers is Not Allowed by Access-Control-Allow-Headers\' Error in Cross-Origin Requests?

Resolving "Request Header Field Access-Control-Allow-Headers is Not Allowed by Access-Control-Allow-Headers" Error

In an attempt to rectify the initial error, headers were added to the request to allow various methods and headers. However, this led to a new error: "Request header field Access-Control-Allow-Origin is not allowed by Access-Control-Allow-Headers."

Understanding CORS and Preflight Requests

The issue stems from the fact that browsers send a preflight OPTIONS request for cross-origin requests with certain content types. By default, Angular sends data with an application/json content type, which triggers this preflight request.

Server Response and Headers

The server must explicitly allow the Access-Control-Allow-Headers header in its response to prevent the error. Otherwise, the browser will interpret the response as not allowing any headers.

Solution: Modifying Content Type or Server Response

To resolve the issue, modify either the client or server configuration:

1. Client-Side (Angular):

Overwrite the default Angular content type to application/x-www-form-urlencoded, which does not trigger a preflight request:

$http.post(url, data, {
    headers : {
        'Content-Type' : 'application/x-www-form-urlencoded; charset=UTF-8'
    }
});

2. Server-Side:

Alternatively, allow the Access-Control-Allow-Headers header on the server. The following response headers must be set:

• Access-Control-Allow-Origin: * or the specific origin (e.g., https://example.com)
• Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS
• Access-Control-Allow-Headers: All headers that the client sends in the request (e.g., Content-Type, Authorization)

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