Use Only FIPS 140-2 Validated MACs

An XCCDF Rule

Description

Limit the MACs to those hash algorithms which are FIPS-approved. The following line in /etc/ssh/sshd_config demonstrates use of FIPS-approved MACs:

MACs hmac-sha2-512,hmac-sha2-256

The man page sshd_config(5) contains a list of supported MACs. The rule is parametrized to use the following MACs: .

warning alert: Warning

The system needs to be rebooted for these changes to take effect.

warning alert: Regulatory Warning

System Crypto Modules must be provided by a vendor that undergoes FIPS-140 certifications. FIPS-140 is applicable to all Federal agencies that use cryptographic-based security systems to protect sensitive information in computer and telecommunication systems (including voice systems) as defined in Section 5131 of the Information Technology Management Reform Act of 1996, Public Law 104-106. This standard shall be used in designing and implementing cryptographic modules that Federal departments and agencies operate or are operated for them under contract. See https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.140-2.pdf To meet this, the system has to have cryptographic software provided by a vendor that has undergone this certification. This means providing documentation, test results, design information, and independent third party review by an accredited lab. While open source software is capable of meeting this, it does not meet FIPS-140 unless the vendor submits to this process.

Rationale

DoD Information Systems are required to use FIPS-approved cryptographic hash functions. The only SSHv2 hash algorithms meeting this requirement is SHA2.

ID: xccdf_org.ssgproject.content_rule_sshd_use_approved_macs
Severity: Medium
References: 1 12 13 15 16 5 8

APO01.06 APO13.01 DSS01.04 DSS05.02 DSS05.03 DSS05.04 DSS05.07 DSS06.02 DSS06.03

3.1.13 3.13.11 3.13.8

CCI-000068 CCI-000803 CCI-000877 CCI-001453 CCI-003123

164.308(b)(1) 164.308(b)(2) 164.312(e)(1) 164.312(e)(2)(i) 164.312(e)(2)(ii) 164.314(b)(2)(i)

4.3.3.5.1 4.3.3.6.6

SR 1.1 SR 1.13 SR 1.2 SR 1.3 SR 1.4 SR 1.5 SR 1.7 SR 1.8 SR 1.9 SR 2.6 SR 3.1 SR 3.5 SR 3.8 SR 4.1 SR 4.3 SR 5.1 SR 5.2 SR 5.3 SR 7.1 SR 7.6

A.10.1.1 A.11.1.4 A.11.1.5 A.11.2.1 A.11.2.6 A.13.1.1 A.13.1.3 A.13.2.1 A.13.2.3 A.13.2.4 A.14.1.2 A.14.1.3 A.6.1.2 A.6.2.1 A.6.2.2 A.7.1.1 A.7.1.2 A.7.3.1 A.8.2.2 A.8.2.3 A.9.1.1 A.9.1.2 A.9.2.1 A.9.2.2 A.9.2.3 A.9.2.4 A.9.2.6 A.9.3.1 A.9.4.1 A.9.4.2 A.9.4.3 A.9.4.4 A.9.4.5

AC-17(2) AC-17(a) CM-6(a) MA-4(6) SC-12(2) SC-12(3) SC-13

PR.AC-1 PR.AC-3 PR.DS-5 PR.PT-4

2.2.7

SRG-OS-000125-GPOS-00065 SRG-OS-000250-GPOS-00093 SRG-OS-000394-GPOS-00174

SLES-15-010270

5.2.14

CCE-91338-4

SV-234826r877395_rule
Updated: about 1 year ago

# Remediation is applicable only in certain platforms
if [ ! -f /.dockerenv ] && [ ! -f /run/.containerenv ]; then

sshd_approved_macs='<xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_sshd_approved_macs" use="legacy"/>'

# 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' <<< "^MACs")

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

# 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 "^MACs\\>" "/etc/ssh/sshd_config"; then
    escaped_formatted_output=$(sed -e 's|/|\\/|g' <<< "$formatted_output")
    LC_ALL=C sed -i --follow-symlinks "s/^MACs\\>.*/$escaped_formatted_output/gi" "/etc/ssh/sshd_config"
else
    if [[ -s "/etc/ssh/sshd_config" ]] && [[ -n "$(tail -c 1 -- "/etc/ssh/sshd_config" || true)" ]]; then
        LC_ALL=C sed -i --follow-symlinks '$a'\\ "/etc/ssh/sshd_config"
    fi
    cce="CCE-91338-4"
    printf '# Per %s: Set %s in %s\n' "${cce}" "${formatted_output}" "/etc/ssh/sshd_config" >> "/etc/ssh/sshd_config"
    printf '%s\n' "$formatted_output" >> "/etc/ssh/sshd_config"
fi

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

- name: XCCDF Value sshd_approved_macs # promote to variable
  set_fact:
    sshd_approved_macs: !!str <xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_sshd_approved_macs" use="legacy"/>
  tags:
    - always
- name: Use Only FIPS 140-2 Validated MACs
  block:

  - name: Check for duplicate values
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)^\s*MACs\s+
      state: absent
    check_mode: true
    changed_when: false
    register: dupes

  - name: Deduplicate values from /etc/ssh/sshd_config
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)^\s*MACs\s+
      state: absent
    when: dupes.found is defined and dupes.found > 1

  - name: Insert correct line to /etc/ssh/sshd_config
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)^\s*MACs\s+
      line: MACs {{ sshd_approved_macs }}
      state: present
      insertbefore: BOF
      validate: /usr/sbin/sshd -t -f %s
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - CCE-91338-4
  - DISA-STIG-SLES-15-010270
  - NIST-800-171-3.1.13
  - NIST-800-171-3.13.11
  - NIST-800-171-3.13.8
  - NIST-800-53-AC-17(2)
  - NIST-800-53-AC-17(a)
  - NIST-800-53-CM-6(a)
  - NIST-800-53-MA-4(6)
  - NIST-800-53-SC-12(2)
  - NIST-800-53-SC-12(3)
  - NIST-800-53-SC-13
  - PCI-DSSv4-2.2.7
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_use_approved_macs

Use Only FIPS 140-2 Validated MACs

Description

Rationale

Remediation - Shell Script

Remediation - Ansible