Vulnerability Discussion

Configuring the "IgnoreUserKnownHosts" setting for the SSH daemon provides additional assurance that remote login via SSH will require a password, even in the event of misconfiguration elsewhere.

OpenSSH uses the first occurrence of a keyword it sees, and drop-in files are read in lexicographical order at the start of the configuration. Red Hat recommends using drop-in files rather than changing base configuration files.

Check

Verify RHEL 10 SSH daemons do not allow known hosts authentication with the following command:

$ sudo /usr/sbin/sshd -dd 2>&1 | awk '/filename/ {print $4}' | tr -d '\r' | tr '\n' ' ' | xargs sudo grep -iH '^\s*ignoreuserknownhosts'
/etc/ssh/sshd_config.d/10-stig.conf:IgnoreUserKnownHosts yes

Verify the runtime setting with the following command:

$ sudo sshd -T | grep -i ignoreuserknownhosts
ignoreuserknownhosts yes

If the "IgnoreUserKnownHosts" keyword is not set to "yes" in a drop-in that lexicographically precedes 50-redhat.conf, or if no output is returned, this is a finding.

Fix

Configure RHEL 10 SSH daemons to not allow known hosts authentication.

In "/etc/ssh/sshd_config.d", create a drop file that will lexicographically precede 50-redhat.conf and add the following line:

IgnoreUserKnownHosts yes

Restart the SSH service with the following command for the changes to take effect:

$ sudo systemctl restart sshd.service