The Ubuntu operating system must immediately terminate all network connections associated with SSH traffic at the end of the session or after 10 minutes of inactivity.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-100845	UBTU-18-010416	SV-109949r1_rule		Medium

Description
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 will also free up resources committed by the managed network element. Terminating network connections associated with communications sessions includes, for example, de-allocating associated TCP/IP address/port pairs at the operating system level, and de-allocating networking assignments at the application level if multiple application sessions are using a single operating system-level network connection. This does not mean that the operating system terminates all sessions or network access; it only ends the inactive session and releases the resources associated with that session.

Description

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 will also free up resources committed by the managed network element. Terminating network connections associated with communications sessions includes, for example, de-allocating associated TCP/IP address/port pairs at the operating system level, and de-allocating networking assignments at the application level if multiple application sessions are using a single operating system-level network connection. This does not mean that the operating system terminates all sessions or network access; it only ends the inactive session and releases the resources associated with that session.

STIG	Date
Canonical Ubuntu 18.04 LTS Security Technical Implementation Guide	2020-05-29

Details

Check Text ( C-99709r1_chk )
Verify that all network connections associated with SSH traffic are automatically terminated at the end of the session or after 10 minutes of inactivity. Check that the "ClientAliveInterval" variable is set to a value of "600" or less by performing the following command: # sudo grep -i clientalive /etc/ssh/sshd_config ClientAliveInterval 600 If "ClientAliveInterval" does not exist, is not set to a value of "600" or less in "/etc/ssh/sshd_config", or is commented out, this is a finding.

Check Text ( C-99709r1_chk )

Verify that all network connections associated with SSH traffic are automatically terminated at the end of the session or after 10 minutes of inactivity.

Check that the "ClientAliveInterval" variable is set to a value of "600" or less by performing the following command:

# sudo grep -i clientalive /etc/ssh/sshd_config

ClientAliveInterval 600

If "ClientAliveInterval" does not exist, is not set to a value of "600" or less in "/etc/ssh/sshd_config", or is commented out, this is a finding.

Fix Text (F-106535r1_fix)
Configure the Ubuntu operating system to automatically terminate all network connections associated with SSH traffic at the end of a session or after a 10 minute period of inactivity. Modify or append the following line in the "/etc/ssh/sshd_config" file replacing "[Interval]" with a value of "600" or less: ClientAliveInterval 600 In order for the changes to take effect, the SSH daemon must be restarted. # sudo systemctl restart sshd.service

Fix Text (F-106535r1_fix)

Configure the Ubuntu operating system to automatically terminate all network connections associated with SSH traffic at the end of a session or after a 10 minute period of inactivity.

Modify or append the following line in the "/etc/ssh/sshd_config" file replacing "[Interval]" with a value of "600" or less:

ClientAliveInterval 600

In order for the changes to take effect, the SSH daemon must be restarted.

# sudo systemctl restart sshd.service