Vulnerability Discussion

Automatic session termination addresses the termination of user-initiated logical sessions in contrast to the termination of network connections that are associated with communications sessions (i.e., network disconnect). A logical session (for local, network, and remote access) is initiated whenever a user (or process acting on behalf of a user) accesses an organizational information system. Such user sessions can be terminated (and thus terminate user access) without terminating network sessions.

Session termination terminates all processes associated with a user's logical session except those processes that are specifically created by the user (i.e., session owner) to continue after the session is terminated.

Conditions or trigger events requiring automatic session termination can include, for example, organization-defined periods of user inactivity, targeted responses to certain types of incidents, and time-of-day restrictions on information system use.

This capability is typically reserved for specific Ubuntu operating system functionality where the system owner, data owner, or organization requires additional assurance.

Check

Verify that all network connections associated with SSH traffic automatically terminate after a period of inactivity.

Check that "ClientAliveCountMax" variable is set in "/etc/ssh/sshd_config" file by performing the following command:

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

ClientAliveCountMax 1

If "ClientAliveCountMax" is not set, or not set to "1", or is commented out, this is a finding.

Fix

Configure the Ubuntu operating system to automatically terminate inactive SSH sessions after a period of inactivity.

Modify or append the following line in the "/etc/ssh/sshd_config" file replacing "[Count]" with a value of 1:

ClientAliveCountMax 1

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

# sudo systemctl restart sshd.service