Title

The macOS system must be configured with the SSH daemon ClientAliveInterval option set to 900 or less. (Cat II impact)

Discussion

Terminating an idle session within a short time period reduces the window of opportunity for unauthorized personnel to take control of a communication session. 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 a 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. The SSH daemon "ClientAliveInterval" option must be set correctly. To check the idle timeout setting for SSH sessions, run the following: /usr/bin/grep -r ^ClientAliveInterval /etc/ssh/sshd_config* If "ClientAliveInterval" is not configured or has a value of "0", this is a finding. If "ClientAliveInterval" is not "900" or less, this is a finding. If conflicting results are returned, this is a finding.

Fix Text

To ensure that "ClientAliveInterval" is set correctly, run the following command: /usr/bin/sudo /usr/bin/sed -i.bak 's/.*ClientAliveInterval.*/ClientAliveInterval 900/' /etc/ssh/sshd_config

Additional Identifiers

Rule ID: SV-252456r891341_rule

Vulnerability ID: V-252456

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

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