Modern Linux includes a module called evbug. If it is not blacklisted, then every USB event is logged to dmesg, and, if you have kernel configured to allow dmesg to be invoked with a non-privileged user, and you have a USB keyboard, there is a method for any user to gain access to your root account.

The fix for this is to blacklist evbug. In /etc/modprobe, create a file such as blacklist.conf, that contains:

blacklist evbug

And reboot. This should stop the evbug module from being loaded and thus your USB keyboard input from being printed to dmesg for potential public viewing.