Validating the server input and appending the input to the base directory path would be the best action for the network administrator to take to defend against this type of web attack, because it would:
Check the user input for any errors, malicious data, or unexpected values before processing it by the web application.
Prevent directory traversal by ensuring that the user input is always relative to the base directory path of the web application, and not absolute to the root directory of the web server.
Deny access to any files or directories that are not part of the web application's scope or functionality.