SSH

Google term

openssh 5.3p1 exploit 

In the version of SSH. I found that we can enumerate the user. I will use this to check for valid username for SSH.

OpenSSH 2.3 < 7.7 - Username EnumerationExploit Database

I also got the list of common usernames of SSH in Linux.

yaptest/ssh-usernames.txt at master · pentestmonkey/yaptestGitHub