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os_ssh_server_alive_count_max_configure Set SSH Active Server Alive Maximum to $ODV

NIST macOS Security Compliance Project Sequoia Guidance, Revision 4.0 (be5491f)
All changes in this version
first version loaded macOS

Title

Set SSH Active Server Alive Maximum to $ODV

Control

ret="pass"
for u in $(/usr/bin/dscl . -list /Users UniqueID | /usr/bin/awk '$2 > 500 {print $1}'); do
sshCheck=$(/usr/bin/sudo -u $u /usr/bin/ssh -G . | /usr/bin/grep -c "^serveralivecountmax $ODV")
if [[ "$sshCheck" == "0" ]]; then
ret="fail"
break
fi
done
/bin/echo $ret

Discussion

SSH _MUST_ be configured with an Active Server Alive Maximum Count set to $ODV. Terminating an idle 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 session or an incomplete login attempt will also free up resources committed by the managed network element.

NOTE: /etc/ssh/ssh_config will be automatically modified to its original state following any update or major upgrade to the operating system.

NOTE: On macOS 15.2 and higher the SSH configuration can be reset to the macOS default by running /usr/libexec/reset-ssh-configuration.

Related controls

sc-10

Fix

[source,bash]
----
include_dir=$(/usr/bin/awk '/^Include/ {print $2}' /etc/ssh/ssh_config | /usr/bin/tr -d '*')

ssh_config=("ServerAliveCountMax $ODV")

ssh_setting=$(echo $ssh_config | /usr/bin/cut -d " " -f1)
/usr/bin/grep -qEi "^$ssh_setting" "${include_dir}01-mscp-ssh.conf" && /usr/bin/sed -i "" "s/^$ssh_setting.*/${ssh_config}/" "${include_dir}01-mscp-ssh.conf" || echo "$ssh_config" >> "${include_dir}01-mscp-ssh.conf"
for u in $(/usr/bin/dscl . list /users shell | /usr/bin/egrep -v '(^_)|(root)|(/usr/bin/false)' | /usr/bin/awk '{print $1}'); do
config=$(/usr/bin/sudo -u $u /usr/bin/ssh -Gv . 2>&1)
configfiles=$(echo "$config" | /usr/bin/awk '/Reading configuration data/ {print $NF}'| /usr/bin/tr -d '\r')
configarray=( ${(f)configfiles} )
if ! echo $config | /usr/bin/grep -q -i "$ssh_config" ; then
for c in $configarray; do
if [[ "$c" == "/etc/ssh/crypto.conf" ]]; then
continue
fi

/usr/bin/sudo -u $u /usr/bin/grep -qEi "^$ssh_setting" "$c" && /usr/bin/sed -i "" "s/^$ssh_setting.*/${ssh_config}/I" "$c"
if [[ "$c" =~ ".ssh/config" ]]; then
if /usr/bin/grep -qEi "$ssh_setting" "$c" 2> /dev/null; then
old_file=$(cat ~$u/.ssh/config)
echo "$ssh_config" > ~$u/.ssh/config
echo "$old_file" >> ~$u/.ssh/config
fi
fi
done
fi
done

----