I thought Dropbear was pretty much the defacto SSH server for embedded applications, is OpenSSH+musl actually a common combination for these scenarios?

Well, OpenWrt is OpenSSH + musl.

OpenWrt uses Dropbear by default:

https://openwrt.org/docs/guide-user/services/ssh/transfer_dr...

We don’t know that dropbear isn’t vulnerable. The bug here is in musl and OpenSSH is just being a bro by working around it

I wouldn't say the bug is only in musl. Musl is probably the "worst" offender here (at least choose something other than "*"!), but the change in sshd was also a little suspect. i.e. the "let's find a user and take their salt (and hashing algorithm) and use it to compute and compare the hash to avoid timing attacks" change.

I haven't thought it through, but I suspect in an ideal world, sshd shouldn't have accepted the user password regardless of the outcome of crypt() if the user did not exist and it only pretended to go through the motions to avoid a timing attack.

Did look into their patch yesterday(?) which seemed to mitigate this issue which they (rightfully IMHO) consider caused by musl. But it seems their fix is to do a sanity check on the supposed hashed password char length, and thus still theoretically still allowing the possibility of those "!!!INVALID_PASSWORD!!!" suggestions to fall through....

Tried it, doesn't work.

Some use Dropbear, some use openssh.

This vulnerability clearly is not one that affects everyone like the PHP vulnerability did. But it's exposure is still significant and very easy to exploit. As easy as it gets.