The recv_msg_userauth_request function in svr-auth.c in Dropbear through 2018.76 is prone to a user enumeration vulnerability because username validity affects how fields in SSH_MSG_USERAUTH messages are handled, a similar issue to CVE-2018-15473 in an unrelated codebase.
https://old.reddit.com/r/blackhat/comments/97ywnm/openssh_username_enumeration/e4e05n2/
https://matt.ucc.asn.au/dropbear/CHANGES
https://lists.debian.org/debian-lts-announce/2018/08/msg00026.html
http://lists.ucc.gu.uwa.edu.au/pipermail/dropbear/2018q3/002109.html
http://lists.ucc.gu.uwa.edu.au/pipermail/dropbear/2018q3/002108.html