Title

SLEM 5 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 become unresponsive 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 '^\s*clientalivecountmax' /etc/ssh/sshd_config:ClientAliveCountMax 1 If "ClientAliveCountMax" is not set to "1", is commented out, missing, or conflicting results are returned, this is a finding.

Fix Text

Add or modify the following line in the "/etc/ssh/sshd_config" file: ClientAliveCountMax 1 Restart the SSH daemon for the changes to take effect: > sudo systemctl restart sshd.service

Additional Identifiers

Rule ID: SV-261331r996459_rule

Vulnerability ID: V-261331

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

Expert Comments

CCIs tied to check.

Controls tied to check. These are derived from the CCIs shown above.