Comment 1 for bug 1988200

Thank you for taking the time to file a bug report.

I tried reproducing this locally but failed. Here's what I did:

$ lxc launch ubuntu:bionic test-mysql
$ lxc shell test-mysql
# apt update
# cd /tmp
# wget https://launchpad.net/~ubuntu-security-proposed/+archive/ubuntu/ppa/+build/22324306/+files/mysql-server-core-5.7_5.7.36-0ubuntu0.18.04.1_amd64.deb https://launchpad.net/~ubuntu-security-proposed/+archive/ubuntu/ppa/+build/22324306/+files/mysql-server-5.7_5.7.36-0ubuntu0.18.04.1_amd64.deb
# apt install ./mysql-server*.deb
# cat >> /etc/mysql/mysql.conf.d/mysqld.cnf << __EOF__
plugin-load-add=auth_socket.so
auth_socket=FORCE_PLUS_PERMANENT
__EOF__
# systemctl restart mysql.service
# mysql -uroot -e 'select user,plugin,authentication_string from mysql.user;'
+------------------+-----------------------+-------------------------------------------+
| user | plugin | authentication_string |
+------------------+-----------------------+-------------------------------------------+
| root | auth_socket | |
| mysql.session | mysql_native_password | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
| mysql.sys | mysql_native_password | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
| debian-sys-maint | mysql_native_password | *BAC57F6E71D415AD367BD54FBE913C5B896313AD |
+------------------+-----------------------+-------------------------------------------+
# apt install mysql-server-5.7 --only-upgrade -y
...
# mysql -uroot -e 'select user,plugin,authentication_string from mysql.user;'
+------------------+-----------------------+-------------------------------------------+
| user | plugin | authentication_string |
+------------------+-----------------------+-------------------------------------------+
| root | auth_socket | |
| mysql.session | mysql_native_password | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
| mysql.sys | mysql_native_password | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
| debian-sys-maint | mysql_native_password | *BAC57F6E71D415AD367BD54FBE913C5B896313AD |
+------------------+-----------------------+-------------------------------------------+

As you can see, the value of 'authentication_string' is still empty, and I can successfully login as root without providing a password.

Since there is not enough information in your report to begin triage or to
differentiate between a local configuration problem and a bug in Ubuntu, I
am marking this bug as "Incomplete". We would be grateful if you would:
provide a more complete description of the problem, explain why you
believe this is a bug in Ubuntu rather than a problem specific to your
system, and then change the bug status back to "New".

For local configuration issues, you can find assistance here:
http://www.ubuntu.com/support/community