In the my.cnf for our PXC cluster nodes, we have been using root user for wsrep_sst_auth with all the admin privileges that comes with the root account. We know using the root user:password inside the config file might not be the most responsible and safe thing to do, so we are thinking to change this. So my question is, what are all the necessary permissions required for the wsrep_sst_auth user that would make the PXC work effectively without having any issue whatsoever?
It seems you are using Xtrabackup-v2 method for SST.
So if you are using that, you should set RELOAD, LOCK TABLES, REPLICATION CLIENT and SUPER privileges to work correctly.
If you do not want set directly user pass in my.cnf you cant start mysql server with this command:
service mysql start --wsrep-sst-auth=username:password