MySQL: ERROR 2027 (HY000): Malformed packet

It is a mysql client bug, I've searched about it and it is a old auth switch request. Your client it is out of date, using a old protocol communication, now, if it is a Workbench problem too your just the Client, you need to update or downgrade the MySQL Client first and try to run it alone.

Here, it is the same question with a more complete answer: https://dba.stackexchange.com/questions/135343/server-responds-with-empty-packet-during-session-negotiation-resulting-in-client

And, for the new Auth protocol, on connection phase: https://dev.mysql.com/doc/dev/mysql-server/latest/page_protocol_connection_phase.html

You must upgrade the "old_password" hashed password:

SET PASSWORD FOR 'existinguser'@'localhost' = PASSWORD('existingpass');

So you can login in an "old" MySQL server, using a recent Workbench version

If you need to connect to pre-4.1 MySQL servers from later MySQL versions (5.7+), you will need to use "--skip-secure-auth" option from the client. And the client version cannot be newer than v5.7.4 because this option had been removed in 5.7.5. You can download version 5.7.4_m14 from mysql's archive website. For example,

$ mysql -uuser -p -hserver --skip-secure-auth