Possibly public SFTP servers. I have allowed null passwords in the past for anonymous public SFTP and I run Alpine Linux exclusively for my servers both VM's and physical. The shadow package (PAM) did not get installed by default. I installed it but I had to specify it in the installation so there are likely people without it. There is probably a way to find them on shodan.io.
I will activate one for people to play around with for today:
sftp hn@ping.ohftp.org
There's no firewall and no password. Hack away HN. PasswordAuthentication yes
PermitEmptyPasswords yes
cd /pubget so*
Edit: Sure, a small minority presumably run sshd under these conditions. But a small minority times the number of Alpine containers running in the world gets to a large absolute number.
Edit 2: You really don't need to look very hard to find public container examples at million-download scale.
I'm guessing in practice relatively few people are actually exposed. But if they are, it's catastrophic.
It would be super helpful if you would answer a "what, specifically" question with a "this, specifically" itemized list or not answer at all.
This is not helpful.