sshd in OpenSSH before 7.4 allows remote attackers to cause a denial of service (NULL pointer dereference and daemon crash) via an out-of-sequence NEWKEYS message, as demonstrated by Honggfuzz, related to kex.c and packet.c.
https://usn.ubuntu.com/3809-1/
https://security.netapp.com/advisory/ntap-20180423-0003/
https://lists.debian.org/debian-lts-announce/2018/09/msg00010.html
https://lists.debian.org/debian-lts-announce/2018/01/msg00031.html
https://anongit.mindrot.org/openssh.git/commit/?id=28652bca29046f62c7045e933e6b931de1d16737