During these years I tested and configured Ultra VNC software version (UltraVNC_1_2_09) that permit you to connect to other client prompting for remote access, permissions grants and acknowledgement.
Consider that usually standard VNC installation permit you to connect to remote host with or without password
End user is not aware that someone is viewing his desktop raising a big privacy issue.
https://www.uvnc.com/downloads/ultravnc.html