Title

The macOS system must be configured with the SSH daemon ClientAliveCountMax option set to 1. (Cat II impact)

Discussion

Terminating an idle session within a short time 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 or an incomplete logon attempt will also free up resources committed by the managed network element. SSH options ClientAliveInterval and ClientAliveCountMax are used in combination to monitor SSH connections. If an SSH client is deemed unresponsive, sshd will terminate the connection. An example would be if a client lost network connectivity the SSH connection to the server would be unresponsive and therefore sshd would terminate the connection after the ClientAliveCountMax and ClientAliveInterval thresholds have been met. The ClientAliveInterval is a timeout measured in seconds. After which if no data is received from the client, sshd will request a response through the encrypted tunnel from the client. The default is 0, indicating no messages will be sent. The ClientAliveCountMax is the number of client alive messages that can be sent from the server without receiving a reply from the client. If this threshold is met, sshd will terminate the session. Setting the ClientAliveCountMax to 0 disables connection termination.

Check Content

If SSH is not being used, this is not applicable. Verify the macOS system is configured with the SSH daemon "ClientAliveCountMax" option set to "1" with the following command: /usr/bin/grep -r ^ClientAliveCountMax /etc/ssh/sshd_config* If the setting is not "ClientAliveCountMax 1", this is a finding. If conflicting results are returned, this is a finding.

Fix Text

Configure the macOS system to set the SSH daemon "ClientAliveCountMax" option to "1" with the following command: /usr/bin/sudo /usr/bin/sed -i.bak 's/.*ClientAliveCountMax.*/ClientAliveCountMax 1/' /etc/ssh/sshd_config

Additional Identifiers

Rule ID: SV-257163r905122_rule

Vulnerability ID: V-257163

Group Title: SRG-OS-000163-GPOS-00072

Expert Comments

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