def upload_cover(request, book):
    if 'btn-upload-cover' in request.files:
        requested_file = request.files['btn-upload-cover']
        # check for empty request
        if requested_file.filename != '':
            if not current_user.role_upload():
                abort(403)
            ret, message = helper.save_cover(requested_file, book.path)
            if ret is True:
                return True
            else:
                flash(message, category="error")
                return False
    return None
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.