I am creating a chrome extension. I am calling this code in background.js

fetch('http://localhost:5000/receive_html', {
    method: 'GET'
  })
  .then(response => {
    if (!response.ok) {
      return response.text().then(text => {
        throw new Error(`HTTP error! status: ${response.status}, body: ${text}`);
      });
    }
    return response.json();
  })
  .then(data => {
    console.log('Success:', data);
  })
  .catch((error) => {
    console.error('Error:', error);
  });

It’s a simple get request, to the backend. I am making this call to test if the rest is working. But in response to this request, I am getting 403 response from the server. This is the code on the server side.

@app.route('/receive_html', methods=['GET'])
def receive_html():
    print("Received request")
    return jsonify({"message": "HTML received and saved successfully"}), 200

This is the manifest.json for the extension.
{
  "manifest_version": 2,
  "name": "Fuuufuufuf",
  "version": "1.0",
  "description": "BLA Bla bla",
  "permissions": [
    "activeTab",
    "storage"
  ],
  "host_permissions": [
    "http://localhost:5000/*",
    "https://localhost:5000/*"
  ],
  "background": {
    "scripts": ["background.js"],
    "persistent": false
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_title": "Find"
  },
  "content_scripts": [
    {
      "matches": ["<all_urls>"],
      "js": ["content.js"]
    }
  ]
}

I tried to send request to the backend from POSTMAN, and its working fine(i.e. I am getting 200 response). But when the request is sent from the extension, it gives 403 error.. I can’t seem to figure out what the problem is. I suspect it is CORS issue. I tried changing many things in this line of code but nothing seems to help
app = Flask(__name__) CORS(app)