How to return a 401 authentication error from a Flask API

Question

A 401 status code is used to indicate unauthorized access in a web API. How can a Flask API return a 401 authentication error when access is denied?

Answer 1

In a Flask API, returning a 401 Unauthorized status code is essential to indicate that a client request lacks valid authentication credentials. This status code informs the client that authentication is required to access the requested resource.

Implementing a 401 Unauthorized Response

To return a 401 status code in Flask, you can utilize the abort function from the flask module, which allows you to abort a request with a specific HTTP status code. Here's how you can implement it:

from flask import Flask, request, jsonify, abort

app = Flask(__name__)

@app.route('/secure-endpoint')
def secure_endpoint():
    auth = request.headers.get('Authorization')
    if not auth or not validate_token(auth):
        abort(401)
    return jsonify({"message": "Welcome to the secure endpoint!"})

def validate_token(token):
    # Implement your token validation logic here
    return token == "Bearer ValidToken"

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

In this example:

• The secure_endpoint function checks for the presence and validity of the Authorization header in the incoming request.
• If the header is missing or invalid, the abort(401) function is called, causing Flask to return a response with a 401 status code.
• The validate_token function contains the logic to verify the provided token.

Including the WWW-Authenticate Header

According to the HTTP specification, a 401 response should include a WWW-Authenticate header field, which defines the authentication method that should be used to access the requested resource. To include this header in your response, you can create a custom error handler for the 401 status code:

from flask import Flask, request, jsonify, make_response

app = Flask(__name__)

@app.errorhandler(401)
def unauthorized(error):
    response = make_response(jsonify({'error': 'Unauthorized access'}), 401)
    response.headers['WWW-Authenticate'] = 'Basic realm="Login required"'
    return response

@app.route('/secure-endpoint')
def secure_endpoint():
    auth = request.headers.get('Authorization')
    if not auth or not validate_token(auth):
        return unauthorized(None)
    return jsonify({"message": "Welcome to the secure endpoint!"})

def validate_token(token):
    # Implement your token validation logic here
    return token == "Bearer ValidToken"

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

In this modified example:

• An error handler for the 401 status code is registered using the @app.errorhandler(401) decorator.
• The unauthorized function creates a response with a 401 status code and includes the WWW-Authenticate header.
• The secure_endpoint function calls the unauthorized function when authentication fails.

Including the WWW-Authenticate header ensures that clients understand the authentication method required, which is particularly important for browsers and HTTP clients that automatically handle authentication challenges.