Security in API enabling CORS in flask

Tired of seeing the red warning under your browser while running your web app?

Often while developing web apps you might make use of a flask backend api and a javascript frontend.

Developing such applications we must take care of CORS.

CORS is an implementation used by browsers to prevent access from non trusted origins and domains. When a request is made to the backend from the frontend the request headers contains a origin. This origin determines if the request is valid or not , if the origin is allowed by the server it passes through or else it is blocked by the browser.

Enabling CORS in flask api

Let us build a simple api using flask

from flask import Flask, request, jsonify

app = Flask(__name__)


@app.route("/")
def home():
    return jsonify(
        {
            "message": "Home page returned successfully"
        }
    )

@app.route('/delete', methods=['POST'])
def delete_resource():
    # Simulate deletion of a resource
    resource_id = request.form.get('resource_id')
    # Returning a message of deletion
    return jsonify({'message': f'Resource {resource_id} deleted successfully'})

if __name__ == '__main__':
    app.run(debug=True)

On running the api head to localhost:5000

We write a frontend html which sends a request to the backend api

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>CORS Demo</title>
</head>
<body>
    <h1>CORS Demo</h1>
    <label for="resource_id">Resource ID:</label>
    <input type="text" id="resource_id" name="resource_id">
    <button onclick="deleteResource()">Delete Resource</button>
    <script>
        function deleteResource() {
            var resourceId = document.getElementById("resource_id").value;
            var xhr = new XMLHttpRequest();
            xhr.open("POST", "http://localhost:5000/delete", true);
            xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
            xhr.onreadystatechange = function () {
                if (xhr.readyState === 4 && xhr.status === 200) {
                    console.log(xhr.responseText);
                }
            };
            xhr.send("resource_id=" + resourceId); // Pass the resource_id input value
        }
    </script>
</body>
</html>

Navigating to the web page on hitting the form we face an error

ON running the inspect element

To fix this error we install flask cors package

pip3 install flask-cors

Make use of the CORS package to update this part in the codebase

from flask import Flask, request, jsonify
from flask_cors import CORS

app = Flask(__name__)
CORS(app)

Run the frontend once again

On running inspect element

The message appears to be successful.

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How to handle CORS in flask security in api PART 1

Table of contents

Enabling CORS in flask api

How to handle CORS in flask security in api PART 1

Table of contents

Cross-Origin Resource Sharing

Enabling CORS in flask api