The main problem on past months was the OpenSSH problem, as everyone could enter an SSH port without password, due to some bugs on the protocol.

There is a lot of things around as well, https://www.deepdotweb.com/2017/07/22/wikileaks-exposes-cias-ssh-hacks-windows-linux/

The important is to have a Firewall that just blocks anything that comes in without being request by the internal network.

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