The kex_input_kexinit function in kex.c in OpenSSH 6.x and 7.x through 7.3 allows remote attackers to cause a denial of service (memory consumption) by sending many duplicate KEXINIT requests. NOTE: a third party reports that "OpenSSH upstream does not consider this as a security issue."
https://security.netapp.com/advisory/ntap-20180201-0001/
https://security.gentoo.org/glsa/201612-18
https://security.FreeBSD.org/advisories/FreeBSD-SA-16:33.openssh.asc
https://github.com/openssh/openssh-portable/commit/ec165c392ca54317dbe3064a8c200de6531e89ad
http://www.securitytracker.com/id/1037057
http://www.securityfocus.com/bid/93776