The SSH daemon must not allow compression or must only allow compression after successful authentication.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-215136	UBTU-16-030350	SV-215136r610931_rule		Medium

Description
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.

STIG	Date
Canonical Ubuntu 16.04 LTS Security Technical Implementation Guide	2020-12-09

Details

Check Text ( C-16335r285276_chk )
Verify the SSH daemon performs compression after a user successfully authenticates. Check that the SSH daemon performs compression after a user successfully authenticates with the following command: # grep Compression /etc/ssh/sshd_config Compression delayed If the "Compression" keyword is set to "yes", is missing, or the returned line is commented out, this is a finding.

Fix Text (F-16333r285277_fix)
Configure SSH to use 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 daemon must be restarted for the changes to take effect. To restart the SSH daemon, run the following command: # sudo systemctl restart sshd.service