/etc/X11/Xsession.d/60x11-common_localhost is in x11-common 1:7.7+13ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | # This file is sourced by Xsession(5), not executed.
[ -x /usr/bin/xhost ] && [ -x /usr/bin/id ] &&
xhost +si:localuser:`id -un` > /dev/null
|