Ensure access to SSH public host key files is configured

MediumLevel 1Automatedlinux

Description

An SSH public key is one of two files used in SSH public key authentication. In this authentication method, a public key is a key that can be used for verifying digital signatures generated using a corresponding private key. Only a public key that corresponds to a private key will be able to authenticate successfully.

How it is checked

Audit method: Shell command

shell

alt0() { o=$(bash -c '#!/usr/bin/env bash
{
   l_ssh_group=""
   grep -Psiq -- '"'"'^\h*ID_LIKE="?([^#\n\r]+)?\bfedora\b'"'"' /etc/os-release 2>/dev/null && \
      grep -Psq -- '"'"'^ssh_keys\b'"'"' /etc/group 2>/dev/null && l_ssh_group="ssh_keys"
   while IFS= read -r l_key; do
      l_file="${l_key}.pub"
      ssh-keygen -lf "$l_file" &>/dev/null || continue
      while IFS=: read -r l_mode l_owner l_group; do
         if [ $(( l_mode & 0133 )) -gt 0 ] || [ "$l_owner" != "root" ] || \
            { [ "$l_group" != "$l_ssh_group" ] && [ "$l_group" != "root" ];
}; then
            printf '"'"'%s\n'"'"' "\"$l_file\": mode $l_mode owner $l_owner group $l_group"
         fi
      done < <(stat -Lc '"'"'%#a:%U:%G'"'"' "$l_file")
   done < <(sshd -T 2>/dev/null | awk '"'"'$1=="hostkey"{print $2}'"'"')
}' 2>&1); [ -z "$o" ] || { echo "step1: $o" | head -c 300; return 1; }; }; if alt0; then echo compliant; fi

Expected output: compliant

How it is fixed

Fix method: Guidance or infrastructure patch

Run the following script to set the owner, group, and restrict permissions on each SSH public host key file from the running sshd configuration: #!/usr/bin/env bash { while IFS= read -r l_key; do l_file="${l_key}.pub" [ -e "$l_file" ] || continue chown root:root "$l_file" chmod u-x,go-wx "$l_file" done < <(sshd -T 2>/dev/null | awk '$1=="hostkey"{print $2}') }

Properties

Operating systems: almalinux-9
Tags: cis-convertedlinux
Reference: CIS AlmaLinux OS 9 Benchmark v3.0.0 - 5.1.4

Check this host

Run this on a machine you are allowed to audit. The scan is read-only.

shell

tmaegis scan --target linux