SLEM 5 must be configured so that all network connections associated with SSH traffic are terminated after 10 minutes of becoming unresponsive.

Terminating an unresponsive SSH 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 SSH session will also free up resources committed by the managed network element.

Terminating network connections associated with communications sessions includes, for example, deallocating associated TCP/IP address/port pairs at the operating system level and deallocating 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 unresponsive session and releases the resources associated with that session.

Verify the SSH server automatically terminates a user session after the SSH client has been unresponsive for 10 minutes by using the following command:

     > sudo /usr/sbin/sshd -dd 2>&1 | awk '/filename/ {print $4}' | tr -d '\r' | tr '\n' ' ' | xargs sudo grep -iH '^\s*clientaliveinterval'
     /etc/ssh/sshd_config:ClientAliveInterval 600

If "ClientAliveInterval" is not set to "600" or less, is commented out, missing, or conflicting results are returned, this is a finding.

Configure the SSH server to terminate a user session automatically after the SSH client has been unresponsive for 10 minutes.

Add or modify the following line in the "/etc/ssh/sshd_config" file:

ClientAliveInterval 600

The SSH daemon must be restarted for any changes to take effect:

     > systemctl restart sshd.service