Ensure the Default Umask is Set Correctly in login.defs

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Description

To ensure the default umask controlled by /etc/login.defs is set properly, add or correct the UMASK setting in /etc/login.defs to read as follows:

UMASK 027

Rationale

The umask value influences the permissions assigned to files when they are created. A misconfigured umask value could result in files with excessive permissions that can be read and written to by unauthorized users.

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}\n' 'login' 2>/dev/null | grep -q installed; then

var_accounts_user_umask='027'


# Strip any search characters in the key arg so that the key can be replaced without
# adding any search characters to the config file.
stripped_key=$(sed 's/[\^=\$,;+]*//g' <<< "^UMASK")

# shellcheck disable=SC2059
printf -v formatted_output "%s %s" "$stripped_key" "$var_accounts_user_umask"

# If the key exists, change it. Otherwise, add it to the config_file.
# We search for the key string followed by a word boundary (matched by \>),
# so if we search for 'setting', 'setting2' won't match.
if LC_ALL=C grep -q -m 1 -i -e "^UMASK\\>" "/etc/login.defs"; then
    escaped_formatted_output=$(sed -e 's|/|\\/|g' <<< "$formatted_output")
    LC_ALL=C sed -i --follow-symlinks "s/^UMASK\\>.*/$escaped_formatted_output/gi" "/etc/login.defs"
else
    if [[ -s "/etc/login.defs" ]] && [[ -n "$(tail -c 1 -- "/etc/login.defs" || true)" ]]; then
        LC_ALL=C sed -i --follow-symlinks '$a'\\ "/etc/login.defs"
    fi
    printf '%s\n' "$formatted_output" >> "/etc/login.defs"
fi

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

Ansible playbook

The following playbook can be run with Ansible to remediate the issue.

- name: Gather the package facts
  package_facts:
    manager: auto
  tags:
  - DISA-STIG-UBTU-20-010016
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - accounts_umask_etc_login_defs
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
- name: XCCDF Value var_accounts_user_umask # promote to variable
  set_fact:
    var_accounts_user_umask: !!str 027
  tags:
    - always

- name: Check if UMASK is already set
  ansible.builtin.lineinfile:
    path: /etc/login.defs
    regexp: ^(\s*)UMASK\s+.*
    state: absent
  check_mode: true
  changed_when: false
  register: result_umask_is_set
  when: '"login" in ansible_facts.packages'
  tags:
  - DISA-STIG-UBTU-20-010016
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - accounts_umask_etc_login_defs
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy

- name: Replace user UMASK in /etc/login.defs
  ansible.builtin.replace:
    path: /etc/login.defs
    regexp: ^(\s*)UMASK(\s+).*
    replace: \g<1>UMASK\g<2>{{ var_accounts_user_umask }}
  when:
  - '"login" in ansible_facts.packages'
  - result_umask_is_set.found > 0
  tags:
  - DISA-STIG-UBTU-20-010016
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - accounts_umask_etc_login_defs
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy

- name: Ensure the Default UMASK is Appended Correctly
  ansible.builtin.lineinfile:
    create: true
    path: /etc/login.defs
    line: UMASK {{ var_accounts_user_umask }}
  when:
  - '"login" in ansible_facts.packages'
  - result_umask_is_set.found == 0
  tags:
  - DISA-STIG-UBTU-20-010016
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - accounts_umask_etc_login_defs
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy