The macOS system must be configured with the SSH daemon ClientAliveCountMax option set to 0.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-81653	AOSX-13-000721	SV-96367r1_rule		Medium

Description
SSH should be configured to log users out after a 15-minute interval of inactivity and to wait only 30 seconds before timing out logon attempts. Terminating an idle session within a short time period reduces the window of opportunity for unauthorized personnel to take control of a management session enabled on the console or console port that has been left unattended. In addition, quickly terminating an idle session or an incomplete logon attempt will also free up resources committed by the managed network element.

Description

SSH should be configured to log users out after a 15-minute interval of inactivity and to wait only 30 seconds before timing out logon attempts. Terminating an idle session within a short time period reduces the window of opportunity for unauthorized personnel to take control of a management session enabled on the console or console port that has been left unattended. In addition, quickly terminating an idle session or an incomplete logon attempt will also free up resources committed by the managed network element.

STIG	Date
Apple OS X 10.13 Security Technical Implementation Guide	2019-03-08

Details

Check Text ( C-81429r1_chk )
The SSH daemon "ClientAliveCountMax" option must be set correctly. To verify the SSH idle timeout will occur when the "ClientAliveCountMax" is set, run the following command: /usr/bin/sudo /usr/bin/grep ^ClientAliveCountMax /etc/ssh/sshd_config If the setting is not "ClientAliveCountMax 0", this is a finding.

Fix Text (F-88501r1_fix)
To ensure that the SSH idle timeout occurs precisely when the "ClientAliveCountMax" is set, run the following command: /usr/bin/sudo /usr/bin/sed -i.bak 's/.ClientAliveCountMax./ClientAliveCountMax 0/' /etc/ssh/sshd_config