sshd in OpenSSH before 7.3, when SHA256 or SHA512 are used for user password hashing, uses BLOWFISH hashing on a static password when the username does not exist, which allows remote attackers to enumerate users by leveraging the timing difference between responses when a large password is provided.
https://access.redhat.com/errata/RHSA-2017:2029
https://access.redhat.com/errata/RHSA-2017:2563
https://cert-portal.siemens.com/productcert/pdf/ssa-412672.pdf
http://seclists.org/fulldisclosure/2016/Jul/51
https://security.gentoo.org/glsa/201612-18
https://security.netapp.com/advisory/ntap-20190206-0001/
https://www.exploit-db.com/exploits/40113/
https://www.exploit-db.com/exploits/40136/