CVE-2018-15473

A user enumeration vulnerability flaw was found in OpenSSH, though version 7.7. The vulnerability occurs by not delaying bailout for an invalid authenticated user until after the packet containing the request has been fully parsed. The highest threat from this vulnerability is to data confidentiality.

Red Hat Product Security has rated this issue as having Low severity. An attacker could use this flaw to determine whether given usernames exist or not on the server, but no further information is disclosed and there is no availability or integrity impact. A future update may address this issue.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Configuring your firewall to limit the origin and/or rate of incoming ssh connections (using the netfilter xt_recent module) will limit the impact of this attack, as it requires a new TCP connection for each username tested. This configuration also provides some protection against brute-force attacks on SSH passwords or keys.

See the following article for more information on limiting access to SSHD: https://access.redhat.com/solutions/8687