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The SSH daemon ClientAliveCountMax option must be set correctly.


Overview

Finding ID Version Rule ID IA Controls Severity
V-67623 AOSX-11-000721 SV-82113r1_rule Medium
Description
SSH should be configured to log users out after a "15" minute interval of inactivity and to only wait "30" seconds before timing out login 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 login attempt will also free up resources committed by the managed network element.
STIG Date
Apple OS X 10.11 Security Technical Implementation Guide 2018-01-04

Details

Check Text ( C-68189r1_chk )
The SSH daemon "ClientAliveCountMax" option must be set correctly. To ensure 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-73737r1_fix)
In order to make sure 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