Information
An access warning may reduce a casual attacker's tendency to target the system. Access warnings may also aid in the prosecution of an attacker by evincing the attacker's knowledge of the system's private status, acceptable use policy, and authorization requirements.
Solution
Perform the following to implement the prescribed state:
To add text with elevated privileges:
sudo defaults write /Library/Preferences/com.apple.loginwindow LoginwindowText 'your text here'
To remove the text with elevated privileges:
sudo defaults delete /Library/Preferences/com.apple.loginwindow LoginwindowText