Oracle MySQL before 5.7.3, Oracle MySQL Connector/C (aka libmysqlclient) before 6.1.3, and MariaDB before 5.5.44 use the --ssl option to mean that SSL is optional, which allows man-in-the-middle attackers to spoof servers via a cleartext-downgrade attack, aka a "BACKRONYM" attack.
Exploitability
AV:NAC:HPR:NUI:NScope
S:UImpact
C:NI:HA:N5.9/CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N