SV-215124r610931: The Ubuntu operating system must implement DoD-approved encryption to protect the confidentiality of SSH connections. | Canonical Ubuntu 16.04 LTS Security Technical Implementation Guide

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Assessment Title

Rule Text

The Ubuntu operating system must implement DoD-approved encryption to protect the confidentiality of SSH connections.

Description

Discussion Text

Without confidentiality protection mechanisms, unauthorized individuals may gain access to sensitive information via a remote access session. Remote access is access to DoD nonpublic information systems by an authorized user (or an information system) communicating through an external, non-organization-controlled network. Remote access methods include, for example, dial-up, broadband, and wireless. Encryption provides a means to secure the remote connection to prevent unauthorized access to the data traversing the remote access connection (e.g., RDP), thereby providing a degree of confidentiality. The encryption strength of a mechanism is selected based on the security categorization of the information. By specifying a cipher list with the order of ciphers being in a “strongest to weakest” orientation, the system will automatically attempt to use the strongest cipher for securing SSH connections.

How this relates to the STIG checklist: STIG vs SCAP benchmark.

IA Controls

NIST SP 800-53

Not mappedCCI-000068SV-90509V-75829

References

DPMS Target Canonical Ubuntu 16.04 LTS DISA DPMS Target Canonical Ubuntu 16.04 LTS 4011

Checks

DISA STIG (Manual Reference)

Verify the SSH daemon is configured to only implement DoD-approved encryption.

Check the SSH daemon's current configured ciphers by running the following command:

# sudo grep -i ciphers /etc/ssh/sshd_config | grep -v '^#'

Ciphers aes256-ctr,aes192-ctr,aes128-ctr

If any ciphers other than "aes256-ctr", "aes192-ctr", or "aes128-ctr" are listed, the order differs from the example above, the "Ciphers" keyword is missing, or the returned line is commented out, this is a finding.

SCC Automation Reference

The reconstructed evaluation script behind this rule's OVAL check(s) — every module a compound check combines, with this rule's own concrete values and DISA's SCC tool's real per-module mechanism curated from static teardown of SCC releases.

#!/bin/bash
# Reconstructed from oval:mil.disa.stig.ubuntu1604:def:233
# The Ubuntu operating system must implement DoD-approved encryption to protect the confidentiality of SSH connections.
# Per-module read mechanism curated from static teardown of DISA's SCC tool
# (SCAP Compliance Checker) -- see the SCC Automation Reference above.

# The openssh-server package is installed
# dpkginfo_object (dpkginfo_test)
test_1() {
  grep -A1 '^Package: openssh-server$' /var/lib/dpkg/status &>/dev/null
}
# /etc/ssh/sshd_config:Ciphers == DoD-approved ciphers
# textfilecontent54_object (textfilecontent54_test)
test_2() {
  (sed -En 's/^\s*(?i)Ciphers(?-i)[ \t]+([\w\",-]+)[\s]*(?:|(?:#.*))?$/\1/p' '/etc/ssh/sshd_config') | grep -Pq '^(aes256-ctr,aes192-ctr,aes128-ctr)$|^"(aes256-ctr,aes192-ctr,aes128-ctr)"$'
}

if ( test_1 && test_2 ); then
  echo "PASS"
else
  echo "FAIL"
fi

Assessment Script Powered by AISandbox-executed

A standalone Shell script to assess this control automatically. Review before running it in your environment.

Syntax-checked and actually executed in an isolated sandbox (not the real system this checks) — confirmed to run without errors.

#!/bin/sh
# oval:mil.disa.stig.ubuntu1604:def:233

if dpkg -s openssh-server &> /dev/null; then
    if grep -qE '^\\s*(?i)Ciphers[ \\t]+(aes256-ctr,aes192-ctr,aes128-ctr)[\\s]*(?:|(?:#.*))?$' /etc/ssh/sshd_config; then
        echo "PASS"
        exit 0
    else
        echo "FAIL"
        exit 1
    fi
else
    echo "PASS"
    exit 0
fi

Remediation

Configure the Ubuntu operating system to allow the SSH daemon to only implement DoD-approved encryption.

Add the following line (or modify the line to have the required value) to the "/etc/ssh/sshd_config" file (this file may be named differently or be in a different location if using a version of SSH that is provided by a third-party vendor):

Ciphers aes256-ctr,aes192-ctr,aes128-ctr

The SSH daemon must be restarted for the changes to take effect. To restart the SSH daemon, run the following command:

# sudo systemctl restart sshd.service