V-253083

Discussion

Terminating an idle 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, de-allocating 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 inactive session and releases the resources associated with that session. TOSS utilizes /etc/ssh/sshd_config for configurations of OpenSSH. Within the sshd_config the product of the values of "ClientAliveInterval" and "ClientAliveCountMax" are used to establish the inactivity threshold. The "ClientAliveInterval" is a timeout interval in seconds after which if no data has been received from the client, sshd will send a message through the encrypted channel to request a response from the client. The "ClientAliveCountMax" is the number of client alive messages that may be sent without sshd receiving any messages back from the client. If this threshold is met, sshd will disconnect the client. The default setting for "ClientAliveCountMax" is "3." If "ClientAliveInterval is set to "15" and "ClientAliveCountMax" is left at the default, unresponsive SSH clients will be disconnected after approximately 45 seconds. Satisfies: SRG-OS-000126-GPOS-00066, SRG-OS-000163-GPOS-00072, SRG-OS-000279-GPOS-00109

Check Content

Verify all network connections associated with SSH traffic are automatically terminated at the end of the session or after 10 minutes of inactivity, or as long as documented with the information system security officer (ISSO) as an operational requirement.

Check that the "ClientAliveInterval" variable is set to a value of "600" or less and that the "ClientAliveCountMax" is set to "1" by performing the following command:

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

ClientAliveInterval 600
ClientAliveCountMax 1

If "ClientAliveInterval" and "ClientAliveCountMax" do not exist, does not have a product value of "600" or less in "/etc/ssh/sshd_config", or is commented out, this is a finding.