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Use Only FIPS 140-2 Validated Key Exchange Algorithms

An XCCDF Rule

Description

Limit the key exchange algorithms to those which are FIPS-approved. Add or modify the following line in /etc/ssh/sshd_config 

KexAlgorithms ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256
This rule ensures that only the key exchange algorithms mentioned above (or their subset) are configured for use, keeping the given order of algorithms.

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 requirements, 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 key exchange algorithms. The system will attempt to use the first algorithm presented by the client that matches the server list. Listing the values "strongest to weakest" is a method to ensure the use of the strongest algorithm available to secure the SSH connection.

ID
xccdf_org.ssgproject.content_rule_sshd_use_approved_kex_ordered_stig
Severity
Medium
References
Updated



Remediation - Ansible

- name: Configure sshd to use FIPS 140-2 approved key exchange algorithms
  lineinfile:
    path: /etc/ssh/sshd_config
    line: KexAlgorithms ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256
    state: present
    regexp: ^\s*KexAlgorithms\s*

Remediation - Shell Script

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

KEX_ALGOS="ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,\
diffie-hellman-group-exchange-sha256"