Prevent Login to Accounts With Empty Passwor (ubuntu2404)

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Description

If an account is configured for password authentication but does not have an assigned password, it may be possible to log into the account without authentication. Remove any instances of the nullok in /etc/pam.d/common-{password,auth,account,session,session-noninteractive} to prevent logins with empty passwords.

Rationale

If an account has an empty password, anyone could log in and run commands with the privileges of that account. Accounts with empty passwords should never be used in operational environments.

Remediation

Shell script

The following script can be run on the host to remediate the issue.

#!/bin/bash

# Remediation is applicable only in certain platforms
if ( dpkg-query --show --showformat='${db:Status-Status}' 'libpam-runtime' 2>/dev/null | grep -q '^installed$' && dpkg-query --show --showformat='${db:Status-Status}' 'linux-base' 2>/dev/null | grep -q '^installed$' ); then

conf_name=cac_unix
conf_path="/usr/share/pam-configs"

if [ ! -f "$conf_path"/"$conf_name" ]; then
    if [ -f "$conf_path"/unix ]; then
        if grep -q "$(md5sum "$conf_path"/unix | cut -d ' ' -f 1)" /var/lib/dpkg/info/libpam-runtime.md5sums;then
            cp "$conf_path"/unix "$conf_path"/"$conf_name"
            sed -i 's/Priority: [0-9]\+/Priority: 257\
Conflicts: unix/' "$conf_path"/"$conf_name"
            DEBIAN_FRONTEND=noninteractive pam-auth-update
        else
            echo "Not applicable - checksum of $conf_path/unix does not match the original." >&2
        fi
    else
        echo "Not applicable - $conf_path/unix does not exist" >&2
    fi
fi
config_file="/usr/share/pam-configs/cac_unix"
sed -i '/pam_unix\.so/s/nullok//g' "$config_file"

DEBIAN_FRONTEND=noninteractive pam-auth-update

else
    >&2 echo 'Remediation is not applicable, nothing was done'
fi