Title

Ubuntu OS must be configured so that all network connections associated with SSH traffic terminate after becoming unresponsive. (Cat II impact)

Discussion

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 the operating system terminates all sessions or network access; it only ends the unresponsive session and releases the resources associated with that session.

Check Content

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 'clientaliveinterval' /etc/ssh/sshd_config:ClientAliveInterval 600 If "ClientAliveInterval" does not exist, is not set to a value of "600" or less, if conflicting results are returned, is commented out, or is missing, this is a finding.

Fix Text

Configure the SSH server to terminate a user session automatically after the SSH client has been unresponsive for 10 minutes. Note: This setting must be applied in conjunction with RIIM-OS-255040 to function correctly. Add or modify the following line in the "/etc/ssh/sshd_config" file: ClientAliveInterval 600 Restart the SSH daemon for the changes to take effect: $ sudo systemctl restart sshd.service

Additional Identifiers

Rule ID: SV-275621r1147913_rule

Vulnerability ID: V-275621

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

Expert Comments

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