/usr/lib/x86_64-linux-gnu/libXau.so is in libxau-dev 1:1.0.8-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libXau.so.6.0.0
/usr/lib/x86_64-linux-gnu/libXau.so.6.0.0