Vulnerability Discussion

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.

Check

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

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

$ sudo grep -ir 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.
If conflicting results are returned, this is a finding.

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

Restart the SSH daemon for the changes to take effect:

$ sudo systemctl restart sshd.service