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.

Check

Verify the SSH daemon performs compression after a user successfully authenticates 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*compression'

Compression delayed

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

Fix

Configure the SSH daemon to not allow compression.

Uncomment the "Compression" keyword in "/etc/ssh/sshd_config" 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