Vulnerability Discussion

If compression is allowed in an SSH connection prior to authentication, vulnerabilities in the compression software could result in compromise of the system from an unauthenticated connection, potentially with root privileges.

Compression options are:
no - disables compression
delayed - allow compression only after authentication
yes - enables compression before authentication, which can leak sensitive metadata and is not recommended

Check

Verify the SSH daemon disables compression or only performs compression after a user successfully authenticates with the following command:

$ sudo grep -ir compression /etc/ssh/sshd_config /etc/ssh/sshd_config

/etc/ssh/sshd_config:Compression no

If the "Compression" keyword is set to "yes", is missing, or the returned line is commented out, this is a finding.

Fix

Uncomment the "Compression" keyword in "/etc/ssh/sshd_config" (this file may be named differently or be in a different location if using a version of SSH that is provided by a third-party vendor) on the system and set the value to "delayed" or "no":

Compression no

The SSH service must be restarted for changes to take effect:

$ sudo systemctl restart sshd.service