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:

$ sshd -T | grep compression

Compression no

If the "Compression" keyword is set to "yes", this is a finding.

Fix

Configure the SSH daemon to not allow compression.

Add the following line to "/etc/ssh/sshd_config", or uncomment the line and set the value to "delayed" or preferably "no":

Compression no

Alternatively, add the setting to an include file if the line "Include /etc/ssh/sshd_config.d/*.conf" is found at the top of the "/etc/ssh/sshd_config" file:

$ echo 'Compression no' > /etc/ssh/sshd_config.d/40-compression.conf

Restart the SSH daemon for the settings to take effect:

$ systemctl restart sshd.service