Boxes does not prevent other local users from using the SPICE protocols to access the console of virtual machines created using gnome-boxes. Using UNIX domain socket connections by default would be the best solution for this. Automatically generated random passwords do not authenticate the server and allow it to be impersonated by other users because it is usually running on an untrusted port. Verified with gnome-boxes from 3.8, but I don't think this has changed in later versions.
This bug appears to have been reported against 'rawhide' during the Fedora 22 development cycle. Changing version to '22'. More information and reason for this action is here: https://fedoraproject.org/wiki/Fedora_Program_Management/HouseKeeping/Fedora22
Fedora 22 changed to end-of-life (EOL) status on 2016-07-19. Fedora 22 is no longer maintained, which means that it will not receive any further security or bug fix updates. As a result we are closing this bug. If you can reproduce this bug against a currently maintained version of Fedora please feel free to reopen this bug against that version. If you are unable to reopen this bug, please file a new report against the current release. If you experience problems, please add a comment to this bug. Thank you for reporting this bug and we are sorry it could not be fixed.