Detections
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5.1.2 Ensure permissions on SSH private host key files are configured
Information
An SSH private key is one of two files used in SSH public key authentication. In this authentication method, the possession of the private key is proof of identity. Only a private key that corresponds to a public key will be able to authenticate successfully. The private keys need to be stored and handled carefully, and no copies of the private key should be distributed.If an unauthorized user obtains the private SSH host key file, the host could be impersonated
Solution
Run the following script to set mode, ownership, and group on the private SSH host key files:#!/usr/bin/env bash
{
a_output=(); a_output2=(); l_ssh_group_name="$(awk -F: '($1 ~ /^(ssh_keys|_?ssh)$/) {print $1}' /etc/group)"
f_file_access_fix()
{
while IFS=: read -r l_file_mode l_file_owner l_file_group; do
a_out2=()
[ "$l_file_group" = "$l_ssh_group_name" ] && l_pmask="0137" || l_pmask="0177"
l_maxperm="$( printf '%o' $(( 0777 & ~$l_pmask )) )"
if [ $(( $l_file_mode & $l_pmask )) -gt 0 ]; then
a_out2+=(" Mode: \"$l_file_mode\" should be mode: \"$l_maxperm\" or more restrictive"
" updating to mode: :$l_maxperm\"")
if [ "l_file_group" = "$l_ssh_group_name" ]; then
chmod u-x,g-wx,o-rwx "$l_file"
else
chmod u-x,go-rwx "$l_file"
fi
fi
if [ "$l_file_owner" != "root" ]; then
a_out2+=(" Owned by: \"$l_file_owner\" should be owned by \"root\""
" Changing ownership to \"root\"")
chown root "$l_file"
fi
if [[ ! "$l_file_group" =~ ($l_ssh_group_name|root) ]]; then
[ -n "$l_ssh_group_name" ] && l_new_group="$l_ssh_group_name" || l_new_group="root"
a_out2+=(" Owned by group \"$l_file_group\" should be group owned by: \"$l_ssh_group_name\" or \"root\""
" Changing group ownership to \"$l_new_group\"")
chgrp "$l_new_group" "$l_file"
fi
if [ "${#a_out2[@]}" -gt "0" ]; then
a_output2+=(" - File: \"$l_file\"" "${a_out2[@]}")
else
a_output+=(" - File: \"$l_file\""
"Correct: mode: \"$l_file_mode\", owner: \"$l_file_owner\", and group owner: \"$l_file_group\" configured")
fi
done < <(stat -Lc '%#a:%U:%G' "$l_file")
}
while IFS= read -r -d $'0' l_file; do
if ssh-keygen -lf &>/dev/null "$l_file"; then
file "$l_file" | grep -Piq -- 'bopensshh+([^#
r]+h+)?privateh+keyb' && f_file_access_fix
fi
done < <(find -L /etc/ssh -xdev -type f -print0 2>/dev/null)
if [ "${#a_output2[@]}" -le "0" ]; then
printf '%s
' "" " - No access changes required" ""
else
printf '%s
' "" " - Remediation results:" "${a_output2[@]}" ""
fi
}
See Also
Item Details
Audit Name: CIS Ubuntu Linux 24.04 LTS v1.0.0 L1 Server
Category: ACCESS CONTROL, MEDIA PROTECTION
References: 800-53|AC-3, 800-53|AC-5, 800-53|AC-6, 800-53|MP-2, CSCv7|14.6
Plugin: Unix
Control ID: 3a2863f48dcde8900fe31cb1255157305a314e45bd6cf1e66cf76acd56f90831